Son H. Ho

Transient Analysis of Cryogenic Liquid-Hydrogen Storage Tank with Intermittant Forced Circulation

This paper presents the transient analysis of fluid flow and heat transfer in a zero-boil-off cryogenic storage tank of liquid hydrogen. The system includes a tank with a cylindrical wall and oblate spheroidal top and bottom, a heat pipe located along the symmetric axis of the tank, and an active circulator consisting of a pump-motor assembly and a spray nozzle. Whenever the maximum temperature inside the tank reaches the boiling point under the working pressure in the tank, the pump is activated to create a forced flow at the nozzle to cool down the heated fluid. After a preset interval of 1 h, the pump is shut down and goes on standby until the maximum temperature reaches its threshold again, and then the pump starts a new cycle. The transient simulation allows the visualization of flow-field and temperature distributions, as well as the computation of maximum and mean temperatures of the fluid in various stages of the pump cycle. This information reveals insights into the characteristics of the stored liquid hydrogen and can assist in optimizing the location of temperature sensors, which provide input to the control system.

Sensitivity Analysis of Domain Considerations for Numerical Simulations of Film Cooling

The majority of computational fluid dynamics studies for turbine film cooling have employed the Reynolds-Averaged Navier-Stokes equations with various turbulence modeling techniques to achieve closure, most notably the various two equation (k-e or k-ω) models. For computational simulation of film cooling, modeling the entire testing domain with a row of multiple holes while sustaining a sufficiently fine mesh would demand a large number of grid cells and a hefty computational expense. A significant reduction in the computational domain can be and has been achieved without much harm to the overall accuracy of the film cooling prediction. The current study aimed to investigate the necessary domain parameters for reducing the grid cell count without significantly affecting the accuracy of the solution. The Box-Behnken design for response surface methodology was employed to determine the relative influence of each parameter on the cooling effectiveness prediction. The experimental design matrix was executed for multiple blowing ratios (0.5, 1.0, 2.0) to include the effects of the blowing ratio on the computational domain. The work was carried out using a three-dimensional computational fluid dynamics finite volume method with the RANS equations and k-e turbulence model. A cylindrical film cooling hole with a pitch-to-diameter ratio of 3.0, a length-to-diameter ratio of 7.5, and an inclination angle of 35° was studied. The results are compared against existing data in the literature as well as in-house experimental data. The data from each case is compared in terms of spatially-averaged effectiveness. The modeled entrance length was found to be the most important parameter, with the mainflow height a distant second. The size of the modeled plenum was not found to exert any significant influence on the effectiveness results. Explanations are offered for notable trends in the data and conclusions are drawn concerning the grid optimization process.© 2010 ASME

On Introducing Approximate Solution Methods in Theory of Elasticity

This work presents how approximate solution methods were introduced in a graduate level course of Theory of Elasticity. The three methods introduced are the finite difference method, the finite element method, and the boundary element method. All methods are exemplified by the problem of a thick‐walled cylinder subject to internal pressure with an axisymmetric response. Choosing a single problem to introduce the three methods demonstrates accuracy and efficacy of each method.

Three-Dimensional Analysis of Liquid Hydrogen Cryogenic Storage Tank

*† This paper presents a study of fluid flow and heat transfer in a cryogenic storage tank for liquid hydrogen using a three-dimensional model for numerical simulation. The model includes a tank with cylindrical wall and oblate spheroidal top and bottom, a heat pipe located along the axis of the tank, and a pump-nozzle unit that collects fluid at the suction inlet and discharges at its nozzle face onto the cool tip (evaporator) of the heat pipe in order to prevent the fluid to boil off. A steady state condition was assumed. Simulations with different normal speeds of fluid discharged at the nozzle face were run for a parametric analysis. Typical distributions of velocity and temperature are presented. Average speed and maximum temperature for each case are evaluated for assessing mixing and boiling effects, respectively. Simulations using an axi-symmetric model, which represents the case of an array of many pump-nozzle units installed around the heat pipe, were also carried out with the same fluid discharge speeds as those of the three-dimensional cases for comparison. For both models, the results show that as the normal fluid speed at the nozzle increases, the average fluid speed increases whereas the maximum temperature decreases. It is also found that with the same fluid speed discharged normally from the nozzle face the axi-symmetric model gives higher average speed and higher maximum temperature compared to the threedimensional model.

The Influence of Discretization Scheme on Large Eddy Simulations of Discrete Film Cooling Holes

Large Eddy Simulations were performed on a simple angle cylindrical film cooling hole with a 35° inclination angle and a length-to-diameter ratio of 3.5. A density ratio of 1.25 and velocity ratio of 0.80 was employed, yielding a mass flux ratio of 1.0 and momentum flux ratio of 0.80. Three different discretization schemes were used in otherwise identical simulations: bounded central differencing, pure second-order upwind, and pure secondorder central differencing. The results of these three cases are compared with experimental data from open literature studies in terms of surface adiabatic effectiveness, mean temperature and velocity fields, and unsteady turbulence characteristics. The pure central differencing scheme was found to perform the best in this film cooling scenario, while the upwind scheme also preformed well. The bounded central scheme compared poorly with the experimental data and the other two schemes. The present investigation was made in an attempt to further explore the abilities of Large Eddy Simulations to resolve the complex flow structure arising from the injection of a film cooling jet into a turbine hot gas path.

Effect of Using Ceiling Fan on Human Thermal Comfort in Air-Conditioned Space

This paper presents a study of human thermal comfort in an air-conditioned space with the additional use of a ceiling fan. A two-dimensional steady-state problem was solved by using computational fluid dynamics (CFD) simulation. The computational model includes a room with an inlet and an outlet, mounted on two opposite walls, of the air-conditioning system, and a person standing at the middle of the room right under a ceiling fan. A parametric analysis was done based on fourteen simulation cases of conditions combining different locations of the inlet diffuser as well as different vertical forced air velocities produced by the fan. Typical distributions of velocity, temperature, and relative humidity are presented. Predicted mean vote (PMV) and predicted percentage dissatisfied (PPD) are estimated to assess the thermal comfort level of the person. It is found that without the use or with little use of the ceiling fan, thermal comfort is strongly dependent on the location of the inlet diffuser; but as the vertical forced air speed produced by the fan increases, thermal comfort is increasingly dependent on this speed.

Predictions of Relative Humidity and Temperature in an Operating Room

This paper uses airflow simulations to evaluate different ventilation systems on an operating room (OR). This study compares air distribution systems for an operating room by use of computational fluid dynamics (CFD) modeling. The air supply distribution and exhaust arrangements were modeled for a directional air flow system where air moves across the space from the high-pressure supply area to the low pressure exhaust area. Calculations were done to model a typical operating room in a steady-state condition with inclusion of object such as surgical lights, operating table, heat sources such as surgical staff and a patient, side-wall supply grille and exhaust air grilles. The discharge angle for the side-wall supply grille was varied from 0 to 45 degrees. Air return locations were also studied. One and two air exhaust outlet sites inside the surgical suite were considered. In the two-exhaust outlet configuration, one position was close to the floor and the other position was high on the wall. Simulations with combinations of 1:0, 1:0.343, 0.343:1, and 1:1 flow rates between the two return locations were performed. Predictions for the air movement, room temperature, room relative humidity, and concentration of contaminants within the operating room are shown. Analysis of these predictions is discussed. The supply and exhaust conditions of the ventilation air flow are shown to play an important role in the control of air quality. Results show good agreement with experimental data.Copyright

Pneumatically Actuated Diaphragm Single Chamber Micropump

The analysis of a single chamber micropump with a diaphragm driven by a pulse train of external pressure and two passive check valves at inlet and outlet is performed. The model for the micropump is developed based on the fundamentals of fluid mechanics and theory of plates with quasi-static approximation. The simulation results are compared with existing experimental data. They are in good agreement on flow rate and in reasonable agreement on backpressure for low driving frequency up to 3 Hz. It is found that the flow rate increases as the driving frequency increases or the backpressure decreases. The results also predict that a duty cycle of 50% results in the highest flow rate at the same frequency and actuating pressure. Also, a higher actuating pressure yields a higher flow rate. The model presented here can effectively assist the design process of micropumps since the model is directly related to the basic design parameters such as geometric dimensions and material properties.Copyright

Evaluating CFD Modeling of a Thermal Storage Unit

When collecting the energy of the sun for domestic use, there are several options, which include photovoltaic cells and evacuated tube collectors. Arrays of evacuated tube collectors are used to heat water for domestic applications, supplementing the use of a typical hot water heater, while photovoltaic cells transform the sun’s radiation into electricity. The benefit of the tube collectors is that they supplement an appliance that uses a fairly large amount of electricity when compared to others in an average home. However, the collectors cannot operate during the night time and produce more hot water than needed at their peak operation point. A thermal storage unit can be used to even out the conversion of energy throughout the day to solve this problem. This study proposes a system using paraffin wax to store thermal energy collected during the day by melting the wax. The system makes use of a finned heat exchanger, with paraffin wax on the shell side, and glycol on the tube side as the heat transfer fluid. It also includes a separate loop for water to flow through and receive thermal energy from the melted wax. Although the wax used in the study is quite effective at storing thermal energy, it has the problem of low conductivity. So, fins are added to the storage and extraction loops to increase the wax’s thermal conductivity. The fins not only help to melt the wax more quickly but also act as nucleation sites when the wax freezes. Once all the wax is melted, energy can be exchanged from it to heat water. When creating such a unit, it is useful to have simulation tools to guide its design. One such tool is FLUENT, which will be used in this study to create a simulation of part of the unit. The simulation will be compared to experimental data from a prototype unit and evaluated based upon its strengths and weaknesses.Copyright

Numerical and Analytical Simulation of a Thermal Storage Unit for Building Hot Water

The modeling of a latent heat storage unit can be done analytically and numerically, with the two approaches intersecting to help to validate simpler models. In this paper, numerical simulation with a two dimensional FLUENT model is used to determine the accuracy of the fin resistance used in a heat exchanger. This resistance model can then be used to determine which changes within the unit can improve it with a relatively quick calculation. FLUENT tracks the melting and solidification of a material through the enthalpy-porosity method, which is utilized in this simulation. The one dimensional analytical model is based upon the resistance model approach from heat transfer, and can be used to quickly analyze what effect a change in the storage unit has on its operation. In the future, this model will be used to parametrically analyze the storage unit and optimize it in terms of the number of fins, the heat exchange material, and the heat transfer fluid. The analytical model developed shows the same trends as the FLUENT model within an acceptable amount of error on both the melting and solidification processes. The comparison of the analytical and FLUENT models to established analytical models and experimental data shows the fact that several factors need to be taken into consideration when modeling a phase change process, including the shape of the melt front, natural convection within the melted wax, and the fin dimensions of the heat exchanger.