Research

Fiber Supported Droplet Combustion of Biofuels

Microgravity Droplet Combustion of Butanol

Normal Gravity Droplet Combustion of Ethanol (non plant-based)

SolidWorks Model of the Complete Research Apparatus

Triple Contained Polycarbonate Apparatus

SolidWorks Model of the Biofuel Ejection Setup

SolidWorks Model of the Syringe Ejection Setup

Abstract:

The combustion of fuel droplets is a key method for researchers to gain a fundamental understanding of burning rates and droplet combustion processes. However, burning rate data obtained from fuel droplet combustion performed under normal gravity conditions is hindered by buoyancy, which causes droplets to lose spherical symmetry and produces convective flows. In order to reduce the hindering effects of gravity on droplet combustion, a microgravity environment is required. Further, a taut fiber, with a knot or cross-fiber, is used to maintain droplet position while in microgravity. Although significant research involving heptane and fossil fuel fiber supported droplet combustion has been performed in the past, little research has been performed using biofuels, or renewable plant-based fuels. Specifically, our research focuses on acquiring visual data of droplet diameter and flame diameter versus time in order to form a burning rate model of bioethanol and biobutanol. Specified droplet sizes from 2-5 mm in diameter of bioethanol and biobutanol will be accurately ejected onto a taut silicon carbide wire using an automated syringe ejection system. Once each droplet has viscously damped on the fiber and has obtained approximate spherical symmetry, a manually activated igniter will ignite each droplet. Visual data of droplet diameter and flame diameter versus time will be recorded using two separate digital cameras. This data will be used to determine the burning rates for bioethanol and biobutanol through image distance processing by pixel count. The droplet diameter data will be compared against the “d-square” law, which states that the square of the droplet diameter decreases linearly with time. Further, we may compare our burning rate data to that of past Fiber Supported Droplet Combustion experiments in order to determine any differences between biofuel burning rates and non-plant based fuel burning rates. Burning rate data such as this may be used to improve ground-based transportation and fire safety aboard spacecraft by enabling accurate predictions of the burning rates for biofuel droplets.

Objective:

Our aim is to understand the droplet burning characteristics of bioethanol and biobutanol. We plan to determine the burning rates for these two fuels based on the droplet diameter and flame diameter versus time. During each droplet combustion, we plan to take video with one camera that is backlit to obtain the droplet silhouette diameter and one that isn’t backlit for flame diameter. Comparisons will be made between the results of our biofuel droplet combustion to those of past droplet combustion experiments involving fossil fuels in order to better understand the combustion of biofuels.

Hypothesis:

Ground-based droplet combustion experiments contain the problem of buoyancy, caused by gravity, which causes fuel droplets to lose their spherical symmetry and produces convective flows. This loss of spherical symmetry may affect the burning rates as well as making the acquisition of droplet diameter more difficult, if not impossible [1]. In our experiment, bioethanol and biobutanol droplets are ejected onto a silicon carbide wire and then ignited in the absence of gravity to allow for accurate measurement of droplet diameter and flame diameter versus time, providing burning rates for these two biofuels. These data will be based on camera footage that has been calibrated to allow for distance measurement through pixel count of droplet silhouette diameter and flame diameter. Provided that there is a limited amount of biofuel droplet combustion data in scientific literature, this experiment could potentially lead to improvements in ground-based engine combustion as well as fire safety aboard spacecraft when biofuels are involved. The experimental results for varying droplet diameters will be checked against the “d-square law” model of combustion, which “predicts that the square of the droplet diameter decreases linearly with time” (see Figure 1) [2]. Further, the burning rate data for ethanol and butanol will be compared to past experiments, particularly the FSDC-1 and FSDC-2 experiments, to illustrate burning rate differences in heptane, methanol, ethanol, and butanol and to enable modifications of and improvement upon existing theoretical models. We hypothesize that the droplet combustion data of bioethanol and biobutanol will provide valid burning rate data based on the linearly decreasing square of the droplet diameters with time. Once obtained, the burning rate data could prove important for the validation of theoretical models for bioethanol and biobutanol.

Basic Fuel Ejection Setup (CAD Only)

Inner Chamber CAD Assembly

Syringe ejection system pulled back

Syringes ejecting a 5 mm droplet onto the Silicon Carbide fiber

Nichrome wire igniting the droplet

Learning Objectives:

Our goal is to better understand the combustion of bioethanol and biobutanol by studying the droplet and flame extinction rates. We plan to compare the results of our biofuel droplet experiment with past fiber supported droplet experiments involving fossil fuels and non-plant produced alcohols to identify similarities and differences in burning rates. We intend to use these comparisons to gain insights into more efficient engine designs as well as more effective space-based fire safety devices.

Justification for Reduced Gravity Environment Testing:

In order to properly and accurately model and measure droplet combustion rates, it is necessary to perform this experiment with minimal interference to the droplet shape and combustion process while maintaining maximum flexibility with droplet diameter sizes. In 1g conditions, droplets greater than 1mm in diameter begin to lose their spherical shape [4], and this occurrence presents a difficult challenge in droplet combustion modeling and restricts the study of droplets of greater diameters. Furthermore, spherical combustion is nonexistent because buoyancy of hot gases naturally creates convection and directly affects combustion rate of the droplet [5].

Microgravity environments solve both issues by simplifying the geometry in combustion modeling equations [4] and allowing spherically symmetric droplet combustion to exist, thereby allowing data to be collected on how flame rates are affected by droplet diameter size.

The burn time of droplets have been shown to vary with droplet diameter, and consequently, larger droplet diameters necessitate longer periods of microgravity to burn to completion [7]. Although drop towers are able to achieve microgravity conditions necessary to perform these experiments, these towers provide at most 10 seconds of microgravity which limits the maximum droplet size to roughly 3 mm in diameter [6]. With approximately 18 seconds of microgravity provided through a reduced gravity flight, a wider range of droplet diameters can be tested and more accurate results can be obtained in determining how burning rates are affected by droplet diameter and flame diameter for the droplets.

The applications of the experiment span from improving efficiency of combustion engines on Earth and enhancing fire safety aboard spacecraft [4]. In order for these applications to be realized, the experiment must be performed in microgravity to obtain accurate flame rates for the fuel droplets.