Masood Parang

Perturbation solution for spherical and cylindrical solidification by combined convective and radiative cooling

Abstract The problem of inward solidification of a liquid in cylindrical and spherical geometries is considered. Freezing is accomplished at the boundary by both radiative and convective cooling. The liquid is assumed at its melt temperature. The method of strained coordinates is used and the zeroth and first-order solutions are developed for freezing-front location, total freezing time, and temperature profile in the solid. The results are compared with a numerical solution, obtained by using the enthalpy method, for various values of the Stefan number. There is excellent agreement between the solutions for small Stefan number. The advantages of the pertubation method (simplicity, relatively good accuracy) in certain cases are discussed and elaborated.

Experimental and numerical study of solidification and melting of pure materials

Solidification experiments in an enclosure are performed with caprillic acid as the phase-change fluid, and the experimental results for the temperature and solidification front advancement are presented. Caprillic acid, with its less than ambient melting point and low volume reduction on solidification, emerges as a good candidate for solidification experiments. An enthalpy formulation for convection/diffusion phase change in conjunction with an algorithm similar to SIMPLER is used in a formulation developed by Voller for solid-liquid phase change, to generate a numerical solution for lowand high-Prandtl-number fluids. The numerical solution is compared with solidification experimental data. In addition, the numerical solution is also compared with experimental melting data of Wolf and Viskanta for pure tin. The results indicate a good agreement between the experimental data and numerical solutions. This adds confidence to the formulation used in this numerical model.

Unsteady temperature measurement in an enclosed thermoconvectively heated air

Abstract Thermally-induced convection in an enclosed fluid is investigated experimentally and applications and potential importance are briefly discussed. In the experiments described here, air enclosed between parallel plates is subjected to rapid heating from the top (along the gravity vector). The experimentally determined variables included the heated boundary temperature, the cavity pressure, and air temperature. The transient air temperature was measured using an optical method. For this purpose, the temperature along a cross section of the enclosure was found by measuring the refractive effects on a planar laser sheet that traverses the test volume inclined at an angle to the heated wall. The experimental results are compared with the computed results from a numerical model for this process. It is found that the agreement between the two results is very good, especially at shorter times following the initiation of heating.

Microgravity Two-Phase Flow Transition

Two-phase flows under microgravity condition find a large number of important applications in fluid handling and storage, and spacecraft thermal management. Specifically, under microgravity condition heat transfer between heat exchanger surfaces and fluids depend critically on the distribution and interaction between different fluid phases which are often qualitatively different from the gravity-based systems. Heat transfer and flow analysis in two-phase flows under these conditions require a clear understanding of the flow pattern transition and development of appropriate dimensionless scales for its modeling and prediction. The physics of this flow is however very complex and remains poorly understood. This has led to various inadequacies in flow and heat transfer modeling and has made prediction of flow transition difficult in engineering design of efficient thermal and flow systems. In the present study the available published data for flow transition under microgravity condition are considered for mapping. The transition from slug to annular flow and from bubbly to slug flow are mapped using dimensionless variable combination developed in a previous study by the authors. The result indicate that the new maps describe the flow transitions reasonably well over the range of the data available. The transition maps are examined and the results are discussed in relation to the presumed balance of forces and flow dynamics. It is suggested that further evaluation of the proposed flow and transition mapping will require a wider range of microgravity data expected to be made available in future studies.

Two Phase Flow Mapping and Transition Under Microgravity Conditions

In this paper, recent microgravity two-phase flow data for air-water, air-water-glycerin, and air- water-Zonyl FSP mixtures are analyzed for transition from bubbly to slug and from slug to annular flow. It is found that Weber number-based maps are inadequate to predict flow-pattern transition, especially over a wide range of liquid flow rates. It is further shown that slug to annular flow transition is dependent on liquid phase Reynolds number at high liquid flow rate. This effect may be attributed to growing importance of liquid phase inertia in the dynamics of the phase flow and distribution. As a result a new form of scaling is introduced to present data using liquid Weber number based on vapor and liquid superficial velocities and Reynolds number based on liquid superficial velocity. This new combination of the dimensionless parameters seem to be more appropriate for the presentation of the microgravity data and provides a better flow pattern prediction and should be considered for evaluation with data obtained in the future. Similarly, the analysis of bubble to slug flow transition indicates a strong dependence on both liquid inertia and turbulence fluctuations which seem to play a significant role on this transition at high values of liquid velocity. A revised mapping of data using a new group of dimensionless parameters show a better and more consistent description of flow transition over a wide range of liquid flow rates. Further evaluation of the proposed flow transition mapping will have to be made after a wider range of microgravity data become available.

Droplet Entrainment in Two-Phase Flow Under Reduced Gravity

Liquid droplets entrained in concurrent gas flows were investigated under reduced gravity condition produced by parabolic flight paths onboard a NASA C-9 research aircrafts. Reduced gravity results were compared with terrestrial ones. Microgravity tests were performed at two water flow rates,. The results shows that pressure drop in the test section is higher under reduced gravity as compared to normal gravity. Preliminary results indicate that for horizontal or slightly inclined channels in normal gravity, gravity plays a stabilizing role. It is shown that critical gas velocities characterizing the entrainment onset for reduced gravity tests are lower than the corresponding normal gravity results at the same water flow rate.

Challenging Projects in Aerospace and Mechanical Engineering Senior Capstone Design Experience

A significant way to attract engineering students’ interests in engineering and promote their design skills is to implement several exciting NASA programs into the senior-year capstone design experience. In 2002 Mechanical, Aerospace and Biomedical Engineering Department offered two new projects, named “Microgravity” and “Lunar Rover Vehicle”, as senior capstone design projects. Both required participation, on a competitive basis, in two corresponding NASA programs: “The Reduced Gravity Student Flight Opportunities Program” and “The Great Moonbuggy Race”. The experience demonstrated that these programs are very suitable in offering senior students unique opportunities to improve their analytical abilities, develop their design skills, and familiarize them with the process of solving real engineering and scientific problems.

Forced Flow Condensation Simulation and Investigation in Microgravity Saturated Air/Liquid Flow

The interest in forced flow condensation in microgravity is motivated by the design of propulsion and power generation, thermal management and life support space systems. Through the NASA Reduced Gravity Student Flight Opportunity Program, a group of students from the University of Tennessee designed and flew experiments aboard KC-135 aircraft (Johnson Space Center) in March 2003 and March 2004 to investigate simulated forced flow condensation using air-water mixture flow. The experimental apparatus consisted of water loop flow through a clear one-inch pipe. Saturated vapor was simulated by the injection of (water) fog in the test section. The experiments confirmed anticipated slug and annular air/liquid flow patterns under microgravity. The temperature measurements indicated lower water temperature and higher fog exit temperatures at microgravity condition when compared with normal gravity results (water flow rate of 1.2 gallons per minute). At the same time it was observed that for relatively small water flow rate and velocity, heat exchange between air and water streams was larger for reduced gravity conditions as compared to normal gravity conditions. This is found to be related to the phase distribution and flow patterns at different water flow rates and velocities.

Liquid Droplet Entrainment in Reduced and Normal Gravity Two-Phase Flows

New experiments on liquid droplets entrained in concurrent gas flows were performed in March 2007 under reduced gravity condition produced by parabolic flight paths onboard a NASA C-9 research aircraft. Test apparatus was improved based on the results from previous (2006) microgravity tests. Wide range of liquid flow rates and corresponding Reynolds numbers were considered. Reduced gravity results were compared with terrestrial ones. It was found that for relatively low water flow rates the pressure drop is larger under reduced gravity compared with normal gravity results. Conversely, at higher values of water flow rates, the pressure drop is less under microgravity as compared with normal gravity condition. The results under reduced gravity are considered and used to develop and calculate an inception criterion for the onset of liquid droplet entrainment.

NASA Student Programs and Senior Capstone Design Experience

A significant way to attract engineering students, especially aerospace and mechanical engineering majors, to space issues is to implement exciting NASA student programs into the senior-year capstone design experience. Three years ago the University of Tennessee’s Mechanical, Aerospace and Biomedical Engineering Department offered two new projects, named “Microgravity” and “Lunar Rover Vehicle”, as senior capstone design projects. Both require participation, on a competitive basis, in two corresponding NASA programs: “The Reduced Gravity Student Flight Opportunities Program” and “The Great Moonbuggy Race”. Three years of experience have demonstrated that both programs are very suitable in offering senior students unique opportunities to improve their analytical abilities, develop design skills, gain experience in working in multi-disciplinary teams, solve cutting-edge engineering problems, and familiarize themselves with space issues and technical problems.© 2005 ASME