Embedded Files
My research primarily focus on the below mentioned domains:
My research primarily focus on the below mentioned domains:
1- Heat transfer enhancement during natural convection.
1- Heat transfer enhancement during natural convection.
2- Bubble behavior during nucleate pool boiling and CHF limit on different surfaces.
2- Bubble behavior during nucleate pool boiling and CHF limit on different surfaces.
3- Heat transfer analysis during subcooled and saturated nucleate pool boiling.
3- Heat transfer analysis during subcooled and saturated nucleate pool boiling.
4- Critical heat flux analysis on different substrates.
4- Critical heat flux analysis on different substrates.
5- Droplet evaporation from high temperature surfaces.
5- Droplet evaporation from high temperature surfaces.
6- Spray impingement on textured high temperature surfaces.
6- Spray impingement on textured high temperature surfaces.
7- Design and development of higher magnitude heat flux sensors.
The pool boiling involves growth, departure, and collapsing of vapor bubbles on a heated surface. The behavior of these bubbles has a significant impact on the overall heat transfer rate and the occurrence of critical heat flux (CHF), which is a limiting factor in many engineering applications. During pool boiling, the bubble ebullition is divided into three stages namely nucleation, growth, and departure. Nucleation occurs when small vapor bubbles form on the heated surface due to thermal fluctuations. These bubbles then grow in size as more heat is transferred to the liquid, and the temperature of the surrounding fluid increases. Finally, the bubbles detach from the heated surface and rise to the top of the fluid, where they condense and release their latent heat. The heat transfer performance during pool boiling is strongly influenced by the bubble behavior, particularly during the critical heat flux (CHF) regime. CHF is the point at which the heat flux on the heated surface reaches a maximum value, beyond which the heat transfer rate decreases significantly. This occurs when a large number of bubbles coalesce and form a vapor film on the heated surface, which acts as a thermal insulator and reduces the heat transfer. The study of the heat transfer analysis on the expedition of temperature distribution and bubble behavior from nucleation to critical heat flux during pool boiling is discussed. Initially, the study is conducted for natural convection heat transfer from plates in water at different salinity. Subcooled nucleate pool boiling analysis is performed on plates and a correlation for heat transfer coefficient is suggested. Droplet evaporation on high temperature surfaces of different roughness are analysed for water at different temperatures and instantaneous heat transfer coefficient is analysed during the process. The bubble behaviour and critical heat flux analysis on plates and tubes are extensively studied. The effect of subcooling, length, diameter, and width at a given thickness is discussed. An empirical correlation is suggested to predict the magnitude of critical heat flux.
Analysis of droplet evaporation of 100 µl de-ionized water on aluminum and copper substrate is studied. The experiments are performed using both the blocks at temperature varying from 105℃ to 165℃. The test fluid is maintained at different temperatures i.e. 30℃, 50℃, 75℃, and 99.4℃. The instantaneous HTC is calculated and analysed for both the substrates at the different surfaces and test fluid temperatures. It is observed that contact angle decreases with increase in fluid temperature, owing to the fact that with an increase in fluid temperature the intermolecular kinetic energy increases and hence surface energy decreases. The droplets exhibited Leidenfrost behaviour for temperature beyond 145°C with copper and 150°C with aluminum surface. For the same substrate material, a decrease in droplet lifetime is observed when surface temperature increases. Among the different surfaces at the same temperature, the evaporation rate is found to be faster for copper than aluminum. The analysis using the developed GUI shows that the contact area increases with time, reaches a maximum value and then reduces to zero. The apparent peak in the projected area of the droplet is typically higher for copper. When substrate temperature increases for given droplet temperature, instantaneous HTC increases. Also, the instantaneous HTC is lowest for the higher area. The increase in droplet temperature increases the instantaneous HTC. The variation in HTC falls in line with all the aforementioned trends. This shows that the instantaneous HTC incorporates the effects of different factors that govern the droplet evaporation mechanism.
The bubble behaviour and critical heat flux during pool boiling maintained at different pool temperatures is studied. The effect of length, diameter and aspect ratio of horizontally and vertically oriented tubes on CHF is discussed. The experiments are performed in a subcooled and saturated pool under atmospheric pressure. The length of the tubes is varied in the range of 50 mm to 1000 mm. The diameters of 1.2 mm to 9mm are considered in this study. The effect of the longitudinal dimension (length) of the heater on bubble size is analyzed, and it is observed that the coalescence of the bubble increases as the length is increased. The bigger size bubble encapsulates the tube and the onset of burnout occurs at an early stage. The bubble sliding mechanism is observed prior to the occurrence of CHF in horizontally oriented tubes. As the bubbles slides on the tube surface as a result of buoyancy, it is observed that for the tube in vertical orientation the location of CHF is near the foot side of the tube, irrespective of its dimension. The analysis of the tube diameter effect on bubble size is performed. As the tube diameter increases, the sliding gets accompanied by the merging of bubbles, and CHF is reached sooner. It is observed that as the heater length is increased, the CHF value is reduced. But after reaching a length of 400mm and 200 mm in horizontal and vertical orientations respectively, CHF magnitude gets unaffected with a further increment in length for tubes of diameter ranging from 2.5 mm to 9 mm. The inference reveals that with an increase in tube diameter, the CHF magnitude decreases. A significant reduction in CHF magnitude is observed as the tube diameter increases from 1.2 mm to 2.5 mm. A correlation shown in Eq. (3) is suggested to predict the magnitude of critical heat flux. The suggested correlation incorporates the effect of tube longitudinal (length) and lateral (diameter) dimensions, orientation, and pool temperature.
A study on the bubble behaviour and CHF analysis during pool boiling on coated tubes is discussed. The electrodeposition technique is used to coat copper over SS 304 tubes of length 100 mm and diameter 2.5 mm – 4.5 mm. The SEM micrographs show that spherical granular copper is deposited with an average grain size of 3.6 µm whereas, the energy dispersive spectroscopy analysis confirms the maximum elemental composition of copper along with 13.9% oxygen. The average roughness measured on the coated tubes of diameters 2.5 mm, 3.5 mm, and 4.5 mm are found to be 0.793 µm, 0.744 µm, and 0.772 µm respectively. The X-ray diffraction study shows that the diffraction patterns peaks corresponding to Cu were detected and matched with JCPDS card no. 003-1018. Further, the contact angle measurement of sessile drop over the coated tubes shows that the surface changes from hydrophilic to hydrophobic which may be used for moisture protection. The thickness of the coating is uniform for all the tubes and varies in the range of 20 ±1 µm. The weight of copper deposited on the tubes of diameters 2.5 mm, 3.5 mm, and 4.5 mm are obtained to be 398 mg, 442 mg, and 503 mg respectively. It is inferred from the analysis of bubble behaviour at different power for the given length and diameter of the coated tube that with an increase in heat flux, the bubble coalescence occurs, and bubble size increases. The CHF analysis on tubes shows that with an increase in diameter, the magnitude of CHF decreases for both coated and uncoated tubes. It is further observed that burnout occurs on coated tubes earlier than on normal SS 304 tubes. It is inferred from the study that at lower heat flux values, more bubble nucleates in coated tubes as compared to normal SS 304 tubes. This enhances the heat transfer performance of the system by providing higher heat release rate. So, for phase change heat transfer at lower heat flux values, coated tubes can be implemented. The CHF and bubble behavior analysis on tube bundle of inline, diamond, and staggered arrangement is studied. The SS 304 tube exhibits hydrophilic nature and have uniform roughness. The magnitude of CHF is higher for the inline arrangement and is lowest for staggered. There is a small deviation in the CHF magnitude between diamond arrangement and inline arrangement. It is observed that the burnout on tubes is due to bubble encapsulation and sliding known as the bundle effect.
A study is conducted to analyse the effect of SS 304 flat plate length, width, orientation, and pool liquid temperature on CHF under atmospheric pressure. The length is varied from 50 mm to 300 mm and two different width 10 mm and 20 mm are considered. The plates are mounted in upward facing (0ᵒ), vertical (90ᵒ), and downward-facing (180ᵒ) orientations. The pool boiling regimes under uniform heat flux conditions are investigated based on bubble behaviour. The pool is maintained at 25℃ and saturation temperature corresponding to ambient pressure. Brun out method is used to detect the CHF. Therefore, a new test sections are fabricated for each experiment to obtain CHF data. Analysis of bubble behaviour for explaining the heat transfer mechanism and heater burnout is performed. It is inferred that as plate’s width and length increases, nucleation site density increases. This leads to reduction in CHF magnitude. In horizontally oriented plates, the bubbles rises up due to buoyancy. The bubble coalescence creates resistance to heat transfer from the surface until the bubble is escaped to the pool. This leads to the occurrence of CHF. In vertical plates, bubble slides along the width and escape to the pool. As the bubble grows, it lifts up and the burnout because of coalescing is delayed. So CHF occurs late. In downward orientation plates, the escaping of bubble is difficult so, it merges at a faster rate and burnout occurs earlier than other two orientations. It is observed that as the pool temperature is gradually increased from subcooled to saturation, the magnitude of CHF reduces. The vertical orientation of the plate results in a higher CHF magnitude. The downward-facing plates exhibit the lowest magnitude of CHF. As the length of the heater plate increases, the CHF value reduces. It is also observed that the higher the width lesser the CHF.
The experimental analysis is carried out on how heat is transferred to a pool of water of different salinity varying from 0% to 2% by natural convection from an upward-facing heater surface. For the study, stainless steel (SS 304) plates of different lengths and widths are used. All the heaters are kept under uniform heat flux conditions. The surface temperature of the heater plate is measured using an infrared (IR) camera. During the heat transfer procedure, the temperature distribution along the plate is investigated.
The shadowgraph technique is used to display convection currents that emerge during the operation. The study investigates the impact of the heater's aspect ratio, length, and width on the Nusselt number. It has been found that the Nusselt number increases when the heater aspect ratio is increased. Analysing the aspect ratio solely is not enough to understand this phenomenon. The heat transfer technique is, however, governed by the length and width of the heater. It is observed from the study that as heater length is increased, the average Nusselt number increases. It is observed that the average Nusselt number increases with an increase in the width of the heater and it has more influence on the heat transfer process than length.
The variation of the average Nusselt number with the Rayleigh number follows a power law with an exponent of 1/3. A constant for the power law is suggested from the experimental data.
Page updated
Google Sites
Report abuse