Yinhai Zhu

Three-Dimensional Numerical Simulation on the Laminar Flow and Heat Transfer in Four Basic Fins of Plate-Fin Heat Exchangers

In this paper, four basic fins of the plate-fin heat exchangers, rectangular plain fin, strip offset fin, perforated fin, and wavy fin, are modeled and simulated by taking account of fin thickness, thermal entry effect, and end effect. Three-dimensional numerical simulations on the flow and heat transfer in the four fins are investigated and carried out at laminar flow regime. Validity of the modeling technique is verified by comparing computational results with both corresponding experimental data and three empirical correlations from literatures. Global average Colburn factor (j factor) and friction factor (f factor) and their local 1D streamwise-average distributions along the fins are presented by introducing data reduction method. The heat transfer behaviors in both the developing and developed regions are analyzed by examining variations of the local Nusselt number along the flow direction. It is found that the thermal entry length of the four fins might be expressed in the format of L e =c l Re c2 Pr D h, which has the same form as the one in a circular tube.

Numerical Simulation of Transpiration Cooling for Sintered Metal Porous Strut of the Scramjet Combustion Chamber

The strut structure in a scramjet combustion chamber is used to inject fuel into the main stream. The environment surrounding the strut in the scramjet chamber is supersonic flow at very high temperatures. Thus, the leading edge of the strut is easily ablated due to aerodynamic heating. This study analyzes the effect of a transpiration cooling scheme using a sintered metal porous media surface to protect the strut from ablation. Numerical simulations are used to study the transpiration cooling for different strut structures and coolant conditions. The influences of these parameters on the transpiration cooling of the strut are analyzed for a main stream Mach number of 2.5 and a total temperature of 1700 K. The surface temperature can be reduced to a safe temperature with a coolant mass flow rate through the porous media of 27.5 kg/ m2-s. The coolant flow near the leading edge is most important, with less flow needed downstream.

Injector Head Transpiration Cooling Coupled with Combustion in H2/O2 Subscale Thrust Chamber

Transpiration cooling coupled with combustion was investigated in an H2/O2 liquid rocket thrust chamber with a transpiration-cooled injector plate. A numerical model was developed using the real gas equation of state. The H2 and O2 combustion process was modeled by the eddy dissipation concept model, which includes the detailed chemical reaction mechanisms in turbulent flows. The permeability and the inertia coefficient of the metal mesh, porous media used for the injector plate were obtained experimentally. The simulation results for the porous plate temperatures and H2 fluxes compare well with hot firing experiments, giving reliable predictions of the combustion, flow, and heat transfer processes in the liquid rocket thrust chamber. The model was also used to investigate the effects of the inlet conditions and the plate material on the transpiration cooling. The results show that locations near the chamber wall and the ignition hole on the plate surface have higher temperatures than other locations. The...

Investigation of Combined Transpiration and Opposing Jet Cooling of Sintered Metal Porous Struts

ABSTRACT Struts are used to inject fuel into the supersonic mainstream of scramjet combustion chambers. The leading edge of the strut experiences the maximum temperature due to tremendous aerodynamic heating. This study describes a combined transpiration and opposing jet cooling system for sintered metal porous struts with a jet flowing out of a micro-slit along the stagnation point line against the incoming flow with methane as the coolant. The combined cooling method for the struts is then compared to cooling by the standard transpiration method. The influences of different slit widths and coolant injection conditions on the strut cooling are numerically investigated. The results show that the combined cooling method significantly reduces the maximum strut temperature. The maximum strut temperature decreases but the coolant consumption increases with increasing micro-slit width. Increasing the micro-slit width more effectively balances the increased cooling effectiveness with the increased coolant flow than just increasing the coolant injection pressure. Coolant injection with non-uniform pressures with higher pressure in the front cavity and lower pressure in the back cavity more effectively enhances the cooling effectiveness and reduces the thermal gradient.

Inverse Heat Conduction Problem for Estimating Heat Flux on a Triangular Wall

The method for solving inverse heat conduction problems was used to develop a computational model of the heat flux on the high-temperature wall of an engine. The heat flux on the wall surface was calculated from measured temperatures and used as the objective function for the inverse solution. The conjugate gradient method was used to optimize the solution with the differential equations for the heat conduction discretized by the finite element method. A computational method was then developed to solve the inverse problem in a two-dimensional wall. The calculated temperatures agree well with the measured data. The maximum error in the calculated temperatures was 3% even for a sharp change in the heat flux. The corresponding error in the calculated heat fluxes was less than 18%.

Geometry optimization study of ejector in anode recirculation solid oxygen fuel cell system

Computational Fluid Dynamics (CFD) technique is employed to investigate the effects of four geometry parameters: nozzle divergent part length, nozzle exit position, mixing chamber length and diffuser length on the performance of ejector in the solid oxide fuel cell (SOFC) system. 216 simulation cases are conducted to fully study the influence of four parameters on the ejector performance and steam to carbon ratio (STCR) under a wide range of working conditions. Optimum values of the four parameters can provide a general guideline for design of compact and high-performance ejectors.

Experimental investigation of biomimetic self-pumping and self-adaptive transpiration cooling

Transpiration cooling is an effective way to protect high heat flux walls. However, the pumps for the transpiration cooling system make the system more complex and increase the load, which is a huge challenge for practical applications. A biomimetic self-pumping transpiration cooling system was developed inspired by the process of trees transpiration that has no pumps. An experimental investigation showed that the water coolant automatically flowed from the water tank to the hot surface with a height difference of 80 mm without any pumps. A self-adaptive transpiration cooling system was then developed based on this mechanism. The system effectively cooled the hot surface with the surface temperature kept to about 373 K when the heating flame temperature was 1639 K and the heat flux was about 0.42 MW m-2. The cooling efficiency reached 94.5%. The coolant mass flow rate adaptively increased with increasing flame heat flux from 0.24 MW m-2 to 0.42 MW m-2 while the cooled surface temperature stayed around 373 K. Schlieren pictures showed a protective steam layer on the hot surface which blocked the flame heat flux to the hot surface. The protective steam layer thickness also increased with increasing heat flux.

Numerical modeling and analysis of ejector in the proton exchange membrane fuel cell system

Computational Fluid Dynamics (CFD) technique is employed to study the performance of ejector in the fuel delivery line of polymer electrolyte membrane (PEM) fuel cell system. Four different ejector geometries are established and tested under 34 different working conditions. It is found that the humidity at the ejector exit increases with the pressure ratio αP at first then decreases with it. The humidity always decreases with increasing the diameter ratio αD. Results show that there is an inverse relationship between the humidity and the entrainment ratio of the ejector.

Study on thermal stratification in rocket liquid oxygen tank with natural circulation precooling loop

A numerical investigation is performed on the physical field of rocket tank with natural circulation precooling loop based on computational fluid dynamics (CFD) technique. The results show that convection heat transfer is dominant in the bulk above the return inlet position and the heat conduction takes place chiefly below the position. The axial thermal stratification is produced in the main region of tank and the radial stratification appears only in the bottom, and the lowest temperature region moves towards the opposite bottom header region depart from the sidewall near the return position.

Thermal Cracking and Heat Transfer of Hydrocarbon Fuels at Supercritical Pressures in Vertical Tubes

ABSTRACT Regenerative cooling is being used to meet the high cooling requirements of advanced hypersonic flight vehicles using the fuel at supercritical pressures as the coolant. The heat transfer and thermal cracking characteristics vary with the pressure, mass flow rate and heat flux. This work presents an experimental investigation of the thermal cracking and the heat transfer characteristics of supercritical pressure hydrocarbon fuels and their interactions. A proportional production distribution chemical model was developed for the thermal cracking with the pre-exponential factor and activation energy to describe the reactions. Numerical results show good agreement with experimental data at low conversions (<25%). The heat transfer for supercritical pressure fuels with thermal cracking is very different from that without thermal cracking. Thermal cracking near the wall enhances the heat transfer by adding an extra heat sink, while coking and a bubble layer add thermal resistances to reduce the convective heat transfer.