Thermoelectric coolers are preferred in many areas because of their simple mechanism and no need for a refrigerant. In this study, an air-to-water mini thermoelectric cooler system was designed and produced. Experiments were performed by placing different numbers of thermoelectric modules on the liquid-cooling heat sink and applying different voltages. The cooling capacity and COP values of the system under different operating conditions were analyzed and discussed. In addition, the effect of fluid flow rate on system performance and temperature difference between inlet and outlet sections has been presented. The heat transfer and flow behavior of the fluid in the liquid-cooling heat sink were determined using CFD simulation methods. Moreover, the heat loss from the system was tried to be reduced by using extra foam insulation and the results were compared with single foam and the effect of the insulation on the temperature drop inside cooler was discussed. At 0.011 kg s−1 mass flow rate and 12 V voltage conditions, when the number of TE modules is increased from 1 to 3 in the TE cooler, a maximum increase of 35% in cooling load is obtained. Also, if the cases with 3 TE modules and 0.011 kg s−1 flow rate are compared in terms of cooling load, 12 V has 80% higher cooling load than 4 V. According to the numerical results, flow structures that negatively affect the heat transfer interactions and reduce the cooling performance of the TE cooler have been determined in the liquid-cooled heat exchanger. Additionally, a significant decrease in the temperature of the cooling chamber has also been achieved with additional insulation. 

This study involves a series of experiments aimed at assessing the impact of incorporating water-absorbent materials on the productivity of solar stills. It is worth mentioning that the performed experiments examine the effects of both materials and their location inside solar still unit. The solar still used for the experiments comprised a black-painted metal case and a glass layer positioned at a 30◦ angle relative to the horizontal. To enhance evaporation performance, a Peltier cooling module was integrated into the system. Furthermore, to prevent thermal losses and improve efficiency, the solar still case was insulated using Styrofoam material. From a scientific standpoint, the variations in this context can be ascribed to three primary variables as, integration Peltier condensation unit, use of water absorber materials like hemp and high-absorbent papers, and the incorporation of absorber dark colors. Throughout the experiments, constant test conditions were established using an artificial light source. In the reference setup, a total of 25 g of clean purified water was obtained, whereas in Case 5 (utilizing black high-absorbent papers on a triangle metal frame) and Case 6 (employing highabsorbent papers in the form of upright columns), which yielded the most efficient results, the amounts of purified water obtained were approximately 34 and 40 g, respectively. The results indicated a remarkable enhancement in thermal efficiency for Case 5 and Case 6, with approximately 29% and 45% increases, respectively, compared to the reference setup. Furthermore, a cost assessment has been conducted, demonstrating that the productivity gained through the use of internal materials contributes to improvements in cost efficiency. In comparison to the reference case, Case 6 exhibits a cost reduction for cost per liter exceeding 35%. 

In this study, phase change material (PCM) energy storage performance was experimentally investigated for horizontal double-glazing applications. In this context, it was aimed to use PCM for energy storage in horizontal insulating glass applications, and optimize amount of PCM in the glass and the effect of the surface area it occupies on the indoor light level were analyzed. In this regard, the temperature variation in the glass with the amount of PCM was observed and the optimum amount of PCM was evaluated. The experiments were carried out on an insulated test cabin using double-glazed test elements with different PCM ratios as 0 %, 15 %, 30 %, 45 %, and 100 %. An external light source was used to create constant radiation conditions during the experiments and temperature measurements, thermal camera images, as well as light intensity measurements were taken in the test cabin. The results revealed that the increase in the energy storage capacity with the amount of PCM is limited, while the light transmittance decreases continuously due to the solid PCM shading. In other words, as the amount of used PCM increases, the energy storage capacity increases and thermal comfort conditions improve. However, this increase stopped at a certain point and a constant trend was observed. It was concluded that, this situation occurs due to the blocking of the radiation entering the indoor environment and the decrease in the radiation intensity. For the 30 % area ratio, the cooling period was extended by 450 min compared to the reference case, and longer thermal comfort conditions were provided. 

Peltier cooling systems are generally small, portable and simple in operating principle compared to conventional vapor compression cooling systems. For these reasons, Peltier cooling systems are proposed widely to use in the field. In studies on Peltier coolers presented in the literature, equipment such as pumps and fans is used and this causes noise and vibration. In order to resolve these problems and negative effects, a cooling system was designed and manufactured using completely motionless elements. Additionally, the electricity energy need of this thermoelectric cooling chamber is fully met by solar energy. This study was investigated both experimentally and numerically by using computational fluid dynamics (CFD) methods. By designing a special heat exchanger in the produced air-to-air cooler system, it has been proven that, the Peltier system can work effectively without the need for any elements such as fans. Then the effects of the voltage values applied to the system and the number of Peltier modules have been investigated. The experiments were carried out at 4 different voltage values (1.5, 2.0, 2.5 and 3.0 V) and in 3 different Peltier numbers (1, 3 and 5) in consecutive connections. According to the obtained results, the most suitable situation for reducing the temperature of the cooling chamber were obtained as the voltage was at a maximum of 3 V and the Peltier number was maximum (5 Peltiers mode), where the internal temperature of the cooling chamber decreased to 11.28 ◦C. It was observed that, the coefficient of performance (COP) values decreased over time in all experiments. Considering the data taken at the beginning of the experiments, the maximum COP value was obtained as 0.04 when 1 Peltier and 1.5 V voltage were applied 

Energy efficiency and energy conservation are critical issues today, and the use of solar energy is widely recognized as a key solution for achieving energy independence and reducing reliance on fossil fuels. In this study, a water treatment system was installed, and numerous experiments were conducted to improve energy efficiency and investigate the impact of various parameters on water distillation rates. The primary focus of this study was to increase the rate of distillation by enhancing the surface area for rapid evaporation through the use of sponge pieces, and integrating the system with a novel condensation part equipped with a mechanism that sprays water on the external heat exchanger of the thermoelectric module to increase the condensation rate. The study also examined the effects of black color on solar energy absorption, the impact of indoor fan operation on water vapor distribution, the role of insulation in reducing thermal energy losses, and the intensity of solar radiation. The results revealed that the use of sponge pieces and a thermoelectric cooler significantly increased the rate of water distillation, which was about 100.5%. Moreover, the effect of air fans on the distillation rate was observed, which increased the distillation by more than 51.1%. Overall, this study provides valuable insights into the design and optimization of solar-powered water treatment systems, highlighting the importance of energy efficiency and the potential of renewable energy sources in meeting energy needs while reducing impact on the environment. 

The available statistics reveal that a significant portion of the total electrical energy in different communities is used for various heating and cooling purposes and refrigeration in the domestic and industrial sectors, which includes cooling systems in critical areas such as the food and pharmaceutical industries. In this work, air-to-air thermoelectric cooling systems have been analyzed. Experimental and numerical attempts have been made to evaluate the influence of the base thickness size of external heat exchanger on cooling performance of refrigerator. In addition, the cooling system has been operated in different working conditions and the effect of cooling fan velocity on the Coefficient of Performance (COP) value was presented. It was found revealed that, the temperature of the cooling chamber for 4.5 mm thickness dropped to 11.57 ◦C, while this temperature was 9.93 ◦C for 1.5 mm thickness. Considering the final temperatures, there was a 14% difference by reducing the thickness from 4.5 mm to 1.5 mm for the case of Vair = 7.5 external fan speed. For the numerical approach, ANSYS Fluent 16.0 software was utilized to analyze the structure of airflow inside and outside of the refrigerator in detail. As a result of Computational Fluid Dynamics (CFD) simulation temperature distribution and air velocity contours have been obtained and achieved results were discussed. 

Nanofluids are produced by suspending different solid nano-size materials (metal and nonmetal) in a base liquid and are often used in energy systems to increase thermal performance and heat transfer rate. The main problem observed in nanofluids used in heat transfer applications is their instability. Researchers have developed and proposed some solutions to obtain stable nanofluids. One of the most important solutions, is the nanoparticles surface modification method. In this work, Fe3O4 nanoparticles were subjected to chemical processes and their surfaces were modified. Three different modified nanoparticles were synthesized, which are Fe3O4@SiO2@Si(CH2)3-IM [Cl], Fe3O4@Si(CH2)3-IM [Cl], and Fe3O4@SiO2&Si(CH2)3-IM [Cl] nanoparticles. The nanofluids were prepared in 0.2% Vol. fraction by using the produced particles in base fluid which was distilled water, and stability of nanofluids were observed for 3 months. Nanofluids were subjected to ultrasonication for 3.5 h to obtain homogeneous nanofluid. Not modified water-based Fe3O4 nanofluid completely collapsed in approximately 1 week. In modified nanofluids, although sedimentation occurred, it was observed that a certain amount of the particles remained suspended even after 3 months. The most important analyses in this study are Scanning Electron Microscope, X-Ray Diffraction, and Transmission Electron Microscope. 

Water scarcity is a pressing global issue, as a significant portion of the world's water resources remain salty and unsuitable for human consumption. Solar desalination has emerged as a sustainable solution to address this challenge, utilizing solar energy for water decontamination and purification. This study explores the enhancement of solar stills through the integration of thermoelectric (TE) modules, focusing on their impact on desalination performance. The optimal saltwater levels, TE module performance, and the efficiency of the system are identified in this investigation. The results of this study reveal that the integration of TE unit into solar still systems leads to a substantial increase in water production, achieving an impressive 35% improvement in productivity compared to conventional solar stills. The TE modules act as efficient cooling agents, accelerating the condensation process within the system. This improvement has substantial implications for addressing water scarcity in arid regions and providing an eco-friendly solution for freshwater production. By harnessing solar energy and advanced TE technology, the findings of this study underscore the potential of TE-enhanced solar desalination systems in providing sustainable and reliable sources of fresh water, even in areas with limited access to traditional water resources. The successful integration of TE modules into solar stills suggests a promising avenue for future research and development. Further exploration of these systems, optimization of TE module parameters, and increased utilization of renewable energy sources are all steps that can be taken to improve water production efficiency and contribute to water resource sustainability. 

In this study, the aim was to store solar energy in a sunspace room for energy savings in cold regions by using water-filled tin cans. The energy collected in the water during the sunbathing hours is transferred to the environment in the evening when the ambient air temperature suddenly drops. Additionally, the walls were covered with black material in the sunspace area to absorb maximum solar energy and then the heating performance was evaluated. In addition to experimental studies, ANSYS Fluent software (2022 R1 version) as a computational fluid dynamics (CFD) program has been used to simulate the sunspace domain in analyses. According to the obtained results, while there was a sudden drop in temperature in the ambient air after sunset, it was observed that the water temperatures in the tin cans decreased more slowly. This indicates that heat transfer from the tin cans to the ambient air occurs during the night. In addition, the effect of black surface application was shown and the differences between indoor and outdoor temperatures were evaluated. While the average temperature difference between the indoor and outdoor environment during the sunshine period without the black surface was 4.67°C, this difference increased to 9.53°C when the black surface was applied. The highest energy efficiency was achieved with the usage of the black surface, reaching a notable 58.2%. 

In this study, the heat transfer performance with forced convection of two different water-based nanofluids was investigated by applying an alternating magnetic field in a minichannel. CoFe2O4–water and MnFe2O4–water nanofluids have been prepared at 0.5 vol.% and tested. The tests were carried out in a minichannel under laminar flow conditions in the Reynolds numbers range of 300–1700. Nusselt numbers of each fluid used in the experiments were calculated and compared. At the Reynolds number of 1500, the CoFe2O4–water nanofluid exhibited an increase of 12% compared to pure water, while the MnFe2O4–water nanofluid showed an increase of 4%. The Nusselt number increased in both nanofluids by applying the magnetic field to nanofluids. The highest Nusselt number obtained was 9.35 for the CoFe2O4–water nanofluid in the presence of magnetic field. While this increase was more pronounced at low Reynolds numbers, a lower rate of increase was obtained at high Reynolds numbers. In addition, the use of nanofluids significantly increased the pressure drop compared to the base fluid. While an almost 100% increase in the pressure drop was observed for the CoFe2O4–water nanofluid compared to pure water, the 65% increase for the MnFe2O4–water nanofluid was maximum. At the highest Reynolds numbers, the maximum pressure drops were determined as 3.4 kPa for the CoFe2O4–water nanofluid and 3 kPa for the MnFe2O4–water nanofluid. It was also detected that the friction factor for CoFe2O4–water and MnFe2O4–water nanofluids was 80% and 40% higher, respectively, than for the base fluid. 

In this study, an attempt has been made to increase the efficiency of the thermoelectric refrigerator by designinig a newgeneration finned heat exchanger. Surface extension, which is one of the most applied passive heat transfer enhancement techniques, was applied for this finned heat exchanger. In this application, the heat absorbed from the cooling room is transferred to the external environment more effectively. In addition, by using an external thermoelectric element (which is installed with the secondary heat exchanger), the heat exchanger cools down faster and the heat is transferred to the environment more quickly. The manufactured cooling system was tested experimentally under different working conditions, including natural and forced convection. The effects of air velocity and applied voltage to the external TE module on thermal performance were examined. Additionally, the external finned heat exchanger has been simulated and heat transfer characteristics have been evaluated using computational fluid dynamics. The lowest and highest COP values have been obtained as 0.003 and 0.011, respectively, when the external TE module has been passive. By providing 12 V for the external TE module, the lowest and highest COP values have been observed as 0.0031 and 0.0042, respectively. In addition, the importance of surface extension applications for the efficient operation of thermoelectric elements has been emphasized. 

In this study, it was targeted to enhance the tribological and thermal properties of Ti6Al4V alloys, which were manufactured with three different build orientations and hatch spacing by using the selective laser melting (SLM) method and a traditional method (casting). In addition, the surfaces of the samples produced by these two methods were coated with the TiAlN thin film by using the cathodic arc physical vapor deposition (CAPVD) method. After the experimental investigations, the lowest wear rate was obtained for the 60-90° sample, and the highest microhardness value was measured as ∼1070 HV0.1 for the 90-45° sample. It was specified that the wear rate rose as the hatch spacing increased among the same build orientation Ti6Al4V alloys produced by SLM method. According to thermal analysis results, among the same hatch spacing values, it was determined that as the build orientation value increased, the specific heat capacity and thermal conductivity values decreased. Among the coated samples, the highest thermal conductivity and specific heat capacity values were obtained for casting samples as 5.63 (W/m·K) and 560.4 (J/kg·K), respectively. 

Purpose The purpose of this study is to numerically and experimentally survey the thermal efficiency of a block-type heat exchanger operated in different working conditions by using pure water and two nanofluids as heat transfer fluids. Design/methodology/approach An aluminum block-type heat exchanger integrated with Peltier thermoelectric element was designed and installed to operate in a cycle, and the thermal performance of the heat exchanger, heat transfer rate, Nusselt and heat transfer coefficient variations were examined at different bath water temperatures by using recycled nanofluids. New generation surface-modified Fe 3 O 4 @SiO 2 -mix-(CH 2 ) 3 Cl@Imidazol/water nanofluid was used as heat transfer fluid in the cycle. In addition, CFD simulation was performed using ANSYS/Fluent to investigate the temperature distribution and fluid flow structure in the used heat exchanger. Findings Experiments were carried out by using numerical and experimental methods. In the experiments, the operating conditions such as flow rate, volume fraction of the nanofluid and water bath temperature were changed to find the effect of each parameter on the thermal efficiency. The Reynolds number varied depending on the test conditions, which was calculated in the range of approximately 100 < Re < 350. In addition, Nusselt number and heat transfer coefficient of test fluids were very close to each other. For 0.4% nanofluid, the maximum h value was obtained as 3837.1, when the Reynolds number was measured as 314.4. Originality/value In the scientific articles published in the field of heat exchangers operated by nanofluids, little attention has been paid to the stability of the nanofluids and sedimentation of particles in the base fluids. In addition, in most cases, experiments were implemented using an electrical resistance as a heat source. In this research, stable surface-modified nanofluids were used as heat transfer fluids, and it was found that the Peltier thermoelectric can be used as heat sources with acceptable efficiency in flat-type heat exchangers and even non-circular channels. 

Peltier cooling systems are usually smaller, more portable, and relatively simpler to operate compared to conventional vapor compression cooling systems. For this reason, Peltier cooling systems are widely recommended for use in the field of cooling applications and refrigerators. These cooling systems have relatively low efficiency despite extensive operation. To solve this problem, a Peltier cooling system operated with advanced nanofluid is proposed in this study. In this cooling system, water-based Fe3O4 nanofluids were used to cool the Peltier. In order to obtain high stability in these nanofluids, the nanoparticles were synthesized chemically with surface modification processes (Fe3O4@SiO2@(CH2)3IM). By designing and manufacturing an air-to-nanofluid cooling system, the performance of Peltier cooling system was evaluated and compared to the conventional air-to-water system. The nanofluids were prepared in three different volume concentrations as 0.2%, 0.5% and 1.0% and then were examined at different working conditions. This research has been analyzed using both experimental and numerical methods. Temperature measurements and experimental COP evaluations were made in the cooling chamber. The flow structure and temperature distribution in spiral heat exchanger were closely surveyed and discussed in detail. According to the results obtained, nanofluid volumetric concentrations, inlet temperatures and mass flow rates had a significant effect on the cooling performance of the Peltier systems. It was observed that COP values decreased over time in all experiments and approach zero gradually. 

Heat pump devices have been researched and analyzed from different aspects, which indicates the importance of these devices. In fact, these devices transfer heat energy effectively from one source to another by consuming power. In this work, the importance of source temperature and its effect on system performance and compressor status has been investigated. It is shown that if the temperature of the heat source is low, the refrigerant at the inlet of the compressor will be in the two-phase region, which could cause damage to the compressor. In these cases, the evaporator design can be changed as available solutions or a high-temperature heat source can be provided in the present study. On the other hand, in the two-phase region, calculating the work of the compressor with enthalpy values will be a problem and may cause a computational error in the power consumption of the compressor. The reason for this is that the enthalpy of refrigerant in the compressor cannot be obtained with two properties, i.e., temperature and pressure. This issue has been considered and the rate of computational error has been obtained. R134a refrigerant was used as circulating gas in the used water-to-air heat pump. The results obtained in the experiments performed showed that the COP value of the heat pump increased by 172% when the source temperature increased from 6°C to 34°C. As the source temperature increases, more energy is transferred to the system. This issue raises both high-pressure and low-pressure values. This increase was recorded as 34% for high pressure and 17% for low pressure when the source temperature increased from 6°C to 34°C. 

In this research, pure water and recycled nanofluids (RNF) are utilized as heat transfer fluids in the thermoelectric cooling (TEC) system, and the effects of these fluids on the cooling performance are experimentally examined. In order to prevent nanofluid sedimentation and enhance stability, a surface modification process on Fe₃O₄ particles is performed. With modified Fe₃O₄@SiO₂-mix-(CH₂)₃Cl@Imidazol nanoparticles, water-based nanofluids are prepared at a constant volumetric concentration. This nanofluid is used in a TEC system and recycled. The sonication time is chosen as the experimental parameter in the preparation of RNF. The RNF are subjected to ultrasonication at different time periods, including 3.5, 7, and 14 hours. The temperature drops inside the cooling chamber, coefficient of performance (COP) value of the TEC system, and dimensionless numbers, including Reynolds and Nusselt of nanofluids, are evaluated and discussed in detail. It is determined that the performance of the TEC system can be increased significantly with the usage of nanofluids. Although some deterioration in heat transfer properties is observed for the RNF, these fluids provide a significant improvement in cooling performance compared to pure water. Increasing the nanofluid flow rate increases the cooling chamber performance up to a certain level. Moreover, a significant increase in TEC chamber performance is also achieved by decreasing the temperature of the water bath in the system. 

The most widely used type of heat transfer in applications is convection heat transfer, and both natural and forced convection modes have an effect in total heat transfer to a certain extent, whether any of these modes is neglected or not. In this study, the contribution of natural and forced convection effects to total heat transfer in rectangular microchannels was investigated experimentally for determining the effect of manufacturing process-induced surface roughness in multiple microchannels fabricated using stainless steel material. The experiments were conducted with microchannel heat sinks having three different surface roughness values of 1.1 µm, 1.8 µm, and 3.0 µm, three microchannel widths of 300 µm, 500 µm and 700 µm and two heights of 300 µm and 450 µm. During the manufacturing process, surface roughness values were controlled by changing electro-erosion fabrication parameters, and detailed surface maps were generated for the microchannel heat sinks. In the experiments using pure water as the working fluid, constant heat flux was applied to the bottom surface of the microchannel heat sink. The experiments were conducted in the range of 10-80 Reynolds numbers to ensure mixed convection conditions. In conclusion, it was determined that surface roughness had a significant effect on mixed convective heat transfer. An increase in surface roughness from 1.1 µm to 1.8 µm in microchannel heat sinks with cross-sections of 300 µm × 300 µm, 700 µm × 300 µm, 300 µm × 450 µm, and 700 µm × 450 µm s led to an increase of about 24%, 19%, 17%, and 13%, respectively, in the Nusselt number. Likewise, the increase in surface roughness from 1.8 µm to 3.0 µm resulted in an increase of 17%, 15%, 14%, and 10%, respectively, in the Nusselt number in the same channel cross-sections. In general, an increase in hydraulic diameter led to a reduction in the effect of surface roughness on overall heat transfer. For a constant microchannel width and height, the effect of surface roughness on mixed convection could be observed only with the increase in the Grashof number. Among the eighteen microchannel heat sinks with different geometric parameters, the best heat transfer results were obtained for the smallest microchannel cross-section of 300 µm × 300 µm and the highest roughness value of 3.0 µm. 

This paper aims to study the entropy generation of CoFe2O4−water nanofluids in minichannels numerically. In the study, the Reynolds number range was chosen with five levels as 4000-20000 and the turbulent flow regime was considered. Pure water was chosen as base fluid and CoFe2O4−water nanofluids were in three different particle volume fractions of 0.25%, 0.5%, and 1.0%. Numerical analyzes were performed for two minichannels with different diameters of 1 mm and 2 mm. The effects of the Reynolds number, nanoparticle concentrations, and minichannel diameters on the frictional entropy generation rates, thermal entropy generation rates, total entropy generation rates, and entropy generation number ratios were numerically analyzed. It was observed that frictional and total entropy generation rates increased as thermal entropy generation rates were decreasing with an increase in particle volume fraction. Also, it was determined that mini channel diameters have a strong effect on thermal entropy generation. 

How the particle size and volumetric ratio of silicon carbide (SiC) powder additions will strengthen polymethyl methacrylate (PMMA) is unclear. The purpose of this in vitro study was to optimize the reinforcement parameters of PMMA with SiC powder by using the Taguchi experimental design method. Particle size, volumetric rate, silane coupling rate, and mixing type were determined as parameters that would affect the reinforcement of PMMA with SiC powder. Using the Taguchi L9 orthogonal array, test specimens with different parameter combinations were fabricated and tested. The fracture load (in newtons) of each specimen group was recorded with the 3-point bend test. The thermal conductivity values of 6050-mm and 3-mm-thick rectangular specimens were measured by using the Linseis THB100 thermal conductivity unit. The thermal diffusivity values were then calculated. Thermal analysis indicated improvement in the thermal conductivity of PMMA after reinforcement with SiC. The maximum thermal diffusivity was obtained with 15% SiC powder by volume. Thermal conductivity and flexural strength increased with an increase in particle size. The maximum flexural strength value was obtained with 5% SiC powder by volume. Increasing the particle size of the filler SiC powder resulted in increased thermal conductivity and flexural strength. Increasing the SiC filler powder by volume increased the thermal conductivity of PMMA but reduced its flexural strength. This study helped determine the optimum conditions for the use of SiC powder. Knowledge of the importance of these variables will help in more effective modification of denture base resin with SiC powder to improve heat transfer without adversely affecting strength. 

In this study, the effects of plasma nitriding surface treatment on the structural, tribological and thermal properties of Ti45Nb Beta alloy were investigated experimentally. Plasma nitriding process of Ti45Nb alloy was performed in 25% Ar-75% N2 gas mixture, for treatment times of 1-4 hours at the temperatures of 700-750°C. The tribological tests of the plasma nitrided alloy were performed in dry sliding conditions. Thermal performance tests were applied to the plasma nitrided samples to determine their thermal conductivity and capacity. The TiNbN2 phase was observed in the compound layer formed after the nitriding process, and the compound layer and diffusion layer thicknesses increased depending on the nitriding temperature and duration. The highest microhardness was obtained for the sample nitrided at 750°C for 4 hours and the hardness approximately twice increased compared to the untreated alloy. Due to its high surface hardness and nitride layer with sufficient thickness, the lowest wear rate was obtained for the nitrided sample under these conditions. As a result, the most effective parameter on the wear properties of Ti45Nb alloy after nitriding was determined as the process temperature. After the plasma nitriding process, thermal conductivity, thermal diffusion, and specific heat capacity values decreased due to the formation of a nitride layer on the surface with a relatively lower thermal conductivity compared to the untreated alloy. 

In this study, mixed convection heat transfer characteristics of nanofluid flow in a circular minichannel were investigated experimentally. The effects of the particle volume ratio (0, 0.25, and 0.75%) and Reynolds number (200-60) on heat transfer by mixed convection were investigated for aiding and opposing flow conditions. Water and water based SiO2 nanofluids were used as working fluid in a minichannel with a diameter of 1.9 mm, and constant heat flux was applied to the outer surface of the minichannel. Temperature-dependent thermophysical properties such as thermal conductivity and viscosity, obtained by experimental measurements, were used in heat transfer calculations. Pressure based numerical computations for all experimental cases were also made by using single phase approach. The results were analyzed separately for aiding flow condition in which secondary flows originating from natural convection are in the same direction with the forced flow, and for opposing flow condition in which secondary flows are in the opposite direction with forced flow. For detailed analysis of mixed convection heat transfer, temperature contours and velocity profiles were obtained by the numerical computations which were compared and validated with the experimental results. According to the data obtained, it was determined that the addition of nanoparticles into the pure water increased the Nusselt number by around 21–64% for the aiding flow, and 18–58% for the opposing flow. The heat transfer in the aiding flow was observed to be minimum 4% and maximum 16% higher in comparison with the opposing flow condition. It was concluded that the direction of secondary flows significantly affects heat transfer. 

Increasing the heat transfer in industrial applications is a frequently encountered engineering problem that requires continuous improvement. In recent years, a commonly used method of heat transfer enhancement is the useing nanofluids. Adding nanometer-sized particles into the working fluid is an application that improves the heat transfer performance of the fluid. In this work, thermal performance and pressure drops of SiO2-pure water nanofluids in a circular microchannel with a diameter of 700 μm were investigated experimentally under constant heat flow boundary conditions. In the study, volumetric ratios of nanofluids were 0.2%, 0.4%, 0.8% and 1.2%. These nanofluids were prepared by adding SiO2 nanoparticles of 15 nm particle diameter to purified water. The values of heat transfer coefficient, Nusselt number, pressure drop and friction factor were determined with temperature, flow rate and pressure measurements. In addition, the thermal conductivity and viscosity properties required for thermal performance and pressure drop analysis have been experimentally determined. As a result, it was observed that Nusselt number increased with increasing Reynolds number and volumetric ratio of particle. With the use of nanofluid, the maximum heat transfer enhancement was about 20% at Re = 2160 and 1.2% particle volume fraction compared to pure water. While the friction factor values of all fluids were very close to each other in the high Reynolds numbers, the effect of the volumetric particle ratio on the friction factor became more clear as the Reynolds number decreased. At all volume ratios it was determined that the friction factor value of the nanofluids is higher than the pure water friction factor value. 

In this study, the characteristics of mixed convection heat transfer of nanofluids in circular microchannels with 500 μm diameter were investigated experimentally. In the study, water and water-based SiO2 nanofluids were used as the working fluid, and volumetric particle ratios of the nanofluids were selected as 0.2 and 0.4%. The thermal conductivity and viscosity characterizations of all fluids were performed in the temperature range of 20–60 °C, and the characteristics related to the temperature obtained from the measurements were used in calculations. The effect of the microchannel inclination angle and particle volumetric ratio on the mixed convection heat transfer characteristics was investigated. Upon examining the results, it was revealed that the range of 13–35% of the total heat transfer was generated by the natural convection effects. Increasing the inclination angle of the test section provided an enhancement between 4 and 13% in the total heat transfer. Furthermore, the increase in the volumetric particle ratio increased both forced convection heat transfer and the natural convection heat transfer components. Adding nanosized SiO2 particles into the water caused the total heat transfer to increase from 12 to 14% for 0.2 vol% and from 29 to 32% for 0.4 vol%.