Ting Ma

Numerical Investigations on the Thermohydraulic Performance of Cross-Wavy Channels with Multi-Periodic Boundary Conditions

The multiple periodic boundary conditions were adopted in both the streamwise and transverse directions in a wavelength section of a cross-wave channel to model flow and heat transfer in a primary surface recuperator, as the ratio of amplitude of waviness to channel pitch is greater than 1/4. The channel wall temperature is further modified as a linearly decreasing profile. It is found that as the ratio of the amplitude to channel pitch (A/P) increases, the heat transfer is augmented, but the pressure drop also increases. When the ratio A/P is 0.333, the CW channel has the best overall performance.

Thermal-Hydraulic Performance of Different Discontinuous Fins Used in a Printed Circuit Heat Exchanger for Supercritical CO2

In this study, four discontinuous fin configurations in parallel and staggered arrangements are investigated to classify their effects on the thermal-hydraulic performance of a printed circuited heat exchanger. Meanwhile, the formation mechanism of the flow resistance of supercritical CO2 is studied in an airfoil fin printed circuited heat exchanger. It shows that the fin configurations have little effect on the overall thermal-hydraulic performance when the mass flow rate of supercritical CO2 is low. The flow resistance dramatically increases during the heating process due to velocity increment caused by increased density, but is not significantly affected by the change in dynamic viscosity.

Numerical Study of Internally Finned Bayonet Tubes in a High Temperature Bayonet Tube Heat Exchanger With Inner and Outer Fins

A bayonet tube heat exchanger with fins being employed outside and inside outer tubes of bayonet-elements is recommended to be used in the ultra high temperature environment by our previous research, such as Externally Fired Combined Cycle and syngas production processes. The present study is concerned with the heat transfer and pressure drop characteristics in internally finned bayonet tubes. Two kinds of thermal boundary conditions applied to the model, i.e., constant wall temperature boundary and convective heat transfer boundary, are calculated and compared. Effects of different fluid flow patterns, inlet Reynolds number and tube length on the heat transfer and pressure drop characteristics are performed. Thermal stress is produced due to the large temperature difference between the inner and outer tube under ultra high temperature environment. Considering the safety of the heat exchanger, the alternative designs are suggested that there should be a small spacing between the inner fin and inner tube surface. The effects of various spacing on the heat transfer and pressure drop characteristics are discussed as well. Based on the results, the suitable spacing is proposed.Copyright

Numerical Study of the Effects of Different Buoyancy Models on Supercritical Flow and Heat Transfer

Due to the dramatic changes in physical properties, the flow and heat transfer in supercritical fluid are significantly affected by buoyancy effects, especially when the ratio of inlet mass flux and wall heat flux is relatively small. In this study, the heat transfer of supercritical water in uniformly heated vertical tube is numerically investigated with different buoyancy models which are based on different calculation methods of the turbulent heat flux. The applicabilities of these buoyancy models are analyzed both in heat transfer enhancement and deterioration conditions. The simulation results show that these buoyancy models make few differences and give good wall temperature prediction in heat transfer enhancement condition when the ratio of inlet mass flux and wall heat flux is very small. With the increase of wall heat flux, the accuracy of wall temperature prediction reduces, and the differences between these buoyancy models become larger. No buoyancy model can currently make accurate wall temperature prediction in deterioration condition in this study.Copyright

Two-dimensional Chemical Etching Process Simulation for Printed Circuit Heat Exchanger Channels Based on Cellular Automata Model

ABSTRACT As a new type of heat exchanger, the printed circuit heat exchanger (PCHE) with high compactness can work under high pressure and high temperature, and can be applied to the very high temperature gas nuclear reactors. The chemical etching technology is one of main processes during PCHE fabrication, which determines whether PCHE flow channels can be properly produced according to design. However, many factors can affect the etching quality and the poor etching process can cause a great deal of uncertainty on the precision of stainless steel flow channels. In this paper, a two-dimensional (2-D) cellular automata model (CA) is used to simulate the etching process on stainless steel to examine the change of channel features varying with time under certain etching rates. Moreover, the simulation results are compared with our previous experimental etching data. On the one hand, from the comparison of the simulation results with channels data by spray etching, 2-D CA model can realize the simulation of etching process and display channel features under different etching rates and lateral erosion rates in every time period. On the other hand, from the comparison of the simulation results with channels data by immersion etching under different FeCl3 concentration and etching liquid temperature, 2-D CA model vividly presents channel features under different factors, which can easily and clearly explain the influence of factors on the etching quality, puddling effect and so on. Therefore, the simulation result strengthens the understanding of the etching mechanism and is helpful to realize an efficient and precise PCHE fabrication.

CFD simulation and optimization of fluid flow distribution inside printed circuit heat exchanger headers

ABSTRACT Fluid flow distribution is related to the performance of heat exchangers. Gross flow distribution is often linked to conventional headers and thus jeopardizes the thermo-hydraulic performance. The objective of this paper is to design several configurations of printed circuit heat exchanger headers to enhance its flow uniformity while leveraging its etched nature. Four kinds of inlet headers, i.e., trapezoid header, double header with detail of bypass holes, core integrated header with one layer, and core integrated header with multilayers, are compared. It is found that the traditional headers induce poor flow distributions. Based on the flow characteristics in traditional headers, a novel core integrated header with seven merged channels is proposed. The flow nonuniformity of the heat exchanger with this novel inlet and outlet headers is reduced by 91%, but the pressure drop is increased by 114% compared with the baseline configuration, whereas it shows the best heat transfer performance as well due to the improvement of flow uniformity.

Configuration Optimization of Cross Wave Primary Surface Sheet in the Recuperator Using Particle Swarm Algorithm

Highly efficient and compact primary surface recuperators are widely used in the microturbine system. However, creep deformation may occur in the passages of the primary surface sheet due to its thin thickness and long operating time. In this paper, the stress and creep deformation of cross wave (CW) primary surface sheet operating for 40000h is numerically studied. In order to improve the creep resistance, the configuration optimization of CW primary surface sheet is carried out using the APDL language in the software ANSYS combined with additional particle swarm optimization (PSO) algorithm. With the object function of minimum creep deformation, the optimal configuration of the CW primary surface sheet is recommended. Compared with the baseline design, the von mises stress and total deformation of the optimal configuration are decreased by 55% and 41%, respectively. The results indicate that the procedure based on PSO algorithm has a significant potential to reduce the creep deformation and it is recommended to be used for configuration optimization.Copyright

Effect of Longitudinal Vortex Generator Location on Thermoelectric-Hydraulic Performance of a Single Stage Integrated Thermoelectric Power Generator

Vortex generators have been widely used to enhance heat transfer in various heat exchangers. Out of the two types of vortex generators: Transverse vortex generators (TVGs) and longitudinal vortex generators (LVGs), LVGs have been found to show better heat transfer performance. Past studies have shown that the implementation of these LVGs can be used to improve heat transfer in thermoelectric generator systems. Here a built in module in COMSOL Multiphysics® was used to study the influence of the location of LVGs in the channel on the comprehensive performance of an integrated thermoelectric device (ITED). The physical model under consideration consists of a copper interconnector sandwiched between p-type and n-type semiconductors and a flow channel for hot fluid in the center of the interconnector. Four pairs of, LVGs are mounted symmetrically on the top and bottom surfaces of the flow channel. Thus, using numerical methods, the thermo-electric-hydraulic performance of the ITED with a single module is examined. By fixing the material size D, the fluid inlet temperature Tin, and attack angle β; the effects of the location of LVGs and Reynolds number were investigated on the heat transfer performance, power output, pressure drop and thermal conversion efficiency. The location of LVGs did not have significant effect on the performance of TEGs in the given model. However, the performance parameters show a considerable change with Reynold’s number and best performance is obtained at Reynold number of Re = 500.Copyright

Direct-Coupling Simulation of Thermal-Hydraulic and Stress Analysis in a Cross-Wave Primary Surface Heat Exchanger

Fluid flow with heat transfer in the Cross Wave (CW) primary surface channels may cause an external stress in the plate due to the non-uniform pressure on the plate surface and non-uniform temperature inside the plate. The plate construction can be deformed under this external stress, which will affect the flow of the fluid by changing the channel dimensions and thus affect the temperature fields. To solve the multi-physical field problem, a direct-coupling simulation method of thermal-hydraulic and stress analysis in a Crow-Wave (CW) primary surface heat exchanger is presented in this paper. The method is based on the commercial code STAR-CD and ABAQUS, and an in house procedure is added to accomplish the direct-coupling simulation in a transient process between thermal-hydraulic and stress modules. Due to the complicated simulation of fluid flow and heat transfer in a two-coupled CW fluid channels in which each channel is separated by the plate (original model) in the CFD (Computer Fluid Dynamics) procedure, an alternative CFD model that is different from the original model is constructed to enable the CFD analysis. The detailed geometry of the original mode is generated in the CAE (Computer Aided Engineering) procedure. A data transition program is developed to control and transform analysis data between the CFD model and the CAE model. The coupling relationship of surface pressure, temperature and the material stress in the CW primary surface heat exchanger is uncovered. The results indicates that the high thermal stress and large displacement about 2.26 × 103 MPa of stress and 8.28 × 10−2 mm of displacement are generated at the beginning of time steps. Therefore, more attention should be paid during the starting up and emergency stop process due to the excessive build of stress during the transient conditions.Copyright

Multiscale Simulations of Fluid Flow for Finned Elliptic Tube Heat Exchangers Using Porous Media Approach

A numerical finned elliptic tube heat exchanger (FETHE) model was proposed to investigate the hydrodynamic characteristics of a full-size FETHE by using the porous media approach. A finned elliptic tube heat exchanger was modeled in such a way that the details of the original structure were replaced by a simple geometry, so that the governing equations can be efficiently solved for a wide range of parameters. The first part of the paper reports there-dimensional numerical optimization results for two fins of elliptic tube arrangements, which are validated by direct comparison with experimental measurements with good agreement. The second part of the paper presents different numbers of fins or tubes arrangements to identify this method. The results are reported for air as the external fluid, in the range 1765≤ReL≤12611, where L is the swept length of the fixed volume. The objective is to show the process of heat exchangers being modeled as a porous media and CFD being used in place of a detailed, experimental effort to obtain closure for the model. Apparently, in order to develop a universal fast running computational tool for complicated heat exchangers with multiple parameter, our current work is a step closer to this goal.Copyright