Natural Convection Heat Transfer and Entropy Generation in a Porous Trapezoidal Enclosure Saturated with Power-Law Non-Newtonian Fluids

Abstract

In the present study, natural convection heat transfer and its associated entropy generation in a porous trapezoidal enclosure saturated with a power-law non-Newtonian fluid has been numerically investigated. Horizontal walls of the enclosure are assumed to be adiabatic while the side walls are considered to be kept at a constant temperature. A continuum-based approach is adapted here to model the fluid flow through porous media and the Darcy’s law is modified to account for non-Newtonian rheological behavior of the fluid. The obtained governing equations are discretized using the finite volume method and a detailed parametric study is undertaken to account for the effects of various relevant parameters of the problem on the heat transfer and entropy generation rates. It was shown that the impact of the power-law index on both entropy generation and heat transfer significantly intensifies in a convection-dominated flow regime inside the enclosure, especially for a shear thinning liquid. Moreover, heat transfer rate and entropy generation increase as the sidewall angle is elevated.     

Thermal analysis of plate condensers in presence of flow maldistribution

Flow maldistribution in plate heat exchangers causes deterioration of both thermal and hydraulic performance. The situation becomes more complicated for two-phase flows during condensation where uneven distribution of the liquid to the channels reduces heat transfer due to high liquid flooding. The present study evaluates the thermal performance of falling film plate condensers with flow maldistribution from port to channel considering the heat transfer coefficient inside the channels as a function of channel flow rate. A generalized mathematical model has been developed to investigate the effect of maldistribution on the thermal performance as well as the exit quality of vapor. A wide range of parametric study is presented, which shows the effects of the mass flow rate ratio of cold fluid and two-phase fluid, flow configuration, number of channels and correlation for the heat transfer coefficient. The analysis presented here also suggests an improved method for heat transfer data analysis for plate condensers.     

Lattice Boltzmann method for nanofluid forced convection heat exchange in a porous channel with multiple heated sources

Abstract

The nanofluid forced convection heat exchange in a porous channel within three heated blocks was numerically investigated using the Nonorthogonal multiple-relaxation time lattice Boltzmann method (MRT-LBM). The effects of various parameters such as nanoparticle volume fraction (?), Darcy number (Da) on heat exchange performance and flow phenomena were analyzed when the Pecklel number (Pe), the Prandtl number (Pr), and porosity (ε) were 25, 5.829 and 0.3, respectively. The outcome showed that the mean Nusselt number (Nu) on the surface of heated sources remarkably improved by adding nanoparticles. Furthermore, the forced convection heat exchange of the fluid flow in the mainstream area and the heat conduction in the liquid retention zone had a conspicuous influence on the heat-transfer properties. It is worth noting that the forced convection heat transfer of the fluid flow dominates heat exchange. The simulation showed that the average surface Nusselt number on the heated blocks and the heat exchange performance declined with the increase of the Darcy number.     

The Design of Cold Plates for the Thermal Management of Electronic Equipment

Cold plates, devices used for the thermal management of electronic equipment, consist of a fluid flow space that is bounded by metallic walls. The fluid passages are designed to optimize the heat extraction from the electronics. This paper deals with the fluid flow and heat transfer in cold plates in which both the fluid flow and heat transfer experience periodic variations in the streamwise direction. The motivation for the work was to devise a methodology for dealing with problems that are highly complex and also computationally demanding. The first goal of the work was to transform the combined problems of fluid flow and conjugate heat transfer into one in which the wall heat transfer can be solved separately. The decoupling was achieved by first focusing on the solution of the full conjugate heat transfer problem for a portion of the periodic array. From this solution, heat transfer coefficients were extracted and subsequently employed for the solution of the wall heat conduction problem for the entire cold plate. The second focus was the development of enhancements of the heat transfer performance of cold plates. Consideration was given to manufacturing as well as thermal and fluid flow issues.     

Computational Conjugate Heat Transfer Analysis of a Hybrid Electric Vehicle Inverter

ABSTRACT

A physics-based computational simulation of the heat transfer characteristics of an insulated gate bipolar transistor (IGBT) developmental inverter is reported. The simulation considers the fluid/thermal multiphysics interactions via a conjugate heat transfer analysis. The fluid phase includes air and liquid coolant; the solid phase, where the heat is conducted, includes various solid materials. Numerical solutions of the heat conduction and convection phenomena in and around the IGBT modules and the inverter, built as a three-dimensional computational model, are sought for by using parallel computing. Comparisons with the available experimental data show a satisfactory agreement of the inverter temperature at three power levels under two different coolant flow rates. Detailed examination of the flow field reveals that the design features of the rectangular coolant flow chamber in the heat sink and the small clearance between the tips of the pin fin and the walls lead to an evenly distributed coolant flow around most of the pin fins. The temperature distributions of the pin fins depend highly on their locations relative to the IGBT modules. The findings from the current study can be useful in future efforts to optimize the thermal performance of IGBT inverters.     

Heat transfer in helical channels with two-sided heating in gas flow

Experimental results and analysis of heat transfer from convex and concave walls (surfaces) and average heat transfer from both walls to hydrodynamically and thermally stabilized air flow in rectangular helical channels with two-sided heating over a wide range of flow hydrodynamic (Re = 10³-2 × 10⁵) and geometric parameters (relative curvature D/h = 5-90 and relative width b/h = 2-20) are presented. It is established, that in case of two-sided heating the heat transfer (Nu numbers) from convex and concave walls increases up to 20% in comparison with one-sided heating. Average heat transfer in helical channels increases up to 50% for laminar-vortex flow regime, and up to 20% for turbulent flow regime in comparison with heat transfer in straight flat channel. Correlations are proposed for stabilized heat transfer for different flow regimes.     

Extruded Microchannel-Structured Heat Exchangers

Abstract

In the search for more compact air/liquid heat exchangers, one possibility is to increase the heat transfer coefficient and surface area by a decrease of the size of the fluid channels. A practical example could be seen in the air/water cross-flow heat exchangers used in cars. For such exchangers, minimization of the total volume leads to a very thin structure, with a lot of small and short air channels. We have designed and patented a cross-flow heat transfer surface with microchannels that has such a structure and can be manufactured industrially at a reasonable cost by extrusion either in aluminum or in polymers. The thermo-hydraulic performance of the structure has been simulated using standard correlations and CFD codes, and prototypic structures are under investigation to validate simulations. Compared to classical heat exchangers, our design is superior in flexibility and compactness for air/liquid applications.     

Effects of Distributor Configuration on Flow Maldistribution in Plate-Fin Heat Exchangers

In this paper, an original concept of a design that adds a complementary fluid cavity in the distributor is presented. The experimental investigation of the effects of distributor configuration parameter on the fluid flow maldistribution in the plate-fin heat exchanger is completed. The correlation of the dimensionless flow maldistribution parameter and the Reynolds number is obtained under different distributor configuration parameters. The experimental studies prove that the performance of flow distribution in heat exchangers can be effectively improved by the optimum design of the distributor's configuration parameter. The ratio of the maximum velocity and the minimum velocity in the channels of the plate-fin heat exchanger can drop from 2.57–3.66 to 2.08–2.81 for various Reynolds numbers. The conclusions are of great significance on the optimum structure design of the plate-fin heat exchangers and can effectively improve the performance of the heat exchangers.     

Construction of and Measurements with an Extremely Compact Cross-Flow Heat Exchanger

Abstract

An extremely compact cross-flow heat exchanger is described. The exchanger is constructed by furnace-brazing together a stack of hundreds of stainless-steel sheets. The resulting structure is leak-tight and very strong, but fluid channels as small as 51 μm (0.002 in) are not plugged by excess brazing material. Measurements of heat transfer between water and liquid propylene flowing through the heat exchanger are in excellent agreement with calculations based on exchanger geometry and fluid properties.     

Supersonic cavity flows over concave and convex walls

Supersonic cavity flows are characterized by compression and expansion waves, shear layer, and oscillations inside the cavity. For decades, investigations into cavity flows have been conducted, mostly with flows at zero pressure gradient entering the cavity in straight walls. Since cavity flows on curved walls exert centrifugal force, the features of these flows are likely to differ from those of straight wall flows. The aim of the present work is to study the flow physics of a cavity that is cut out on a curved wall. Steady and unsteady numerical simulations were carried out for supersonic flow through curved channels over the cavity with L/H = 1. A straight channel flow was also analyzed which serves as the base model. The velocity gradient along the width of the channel was observed to increase with increasing the channel curvature for both concave and convex channels. The pressure on the cavity floor increases with the increase in channel curvature for concave channels and decreases for convex channels. Moreover, unsteady flow characteristics are more dependent on channel curvature under supersonic free stream conditions.     

Liquid cooling of a microprocessor: experimentation and simulation of a sub-millimeter channel heat exchanger

Abstract

A heat exchanger dedicated to the cooling of a microprocessor has been designed and realized. It consists of a bottom wall in contact with the processor and a cover that has been dug to a depth of 200?μm on one side and 1?mm on the other. Thus, by turning the cover, the hydraulic diameter of the channel can be changed. Both hydraulic and thermal performances of this heat exchanger have been experimentally tested. Three-dimensional numerical simulations were simultaneously carried out and good agreement was obtained. The influence of the distributor and the collector on the distribution of fluid flow and heat fluxes is emphasized. A new concept of micro-heat exchanger is proposed for the cooling of electronics devices for which wall to fluid heat exchange quality and pumping effect are critical. The ability of a liquid heat exchanger involving a dynamic deformation of one of its walls to cool a microprocessor is investigated. Three-dimensional transient numerical simulations of fluid flow and conjugate heat transfer were performed using commercial software. Effect of geometrical and actuation parameters has been explored, demonstrating the ability of such heat exchanger to simultaneously pump the fluid and enhance the heat transfer.     

Joule heating in magnetohydrodynamic flows in channels with thin conducting walls

Joule heating in liquid metal magnetohydrodynamic flows is investigated with reference to self-cooled liquid metal blankets for tokamaks. Pressure-driven flow of an electrically conducting fluid confined between two parallel, infinite walls with a transverse magnetic field is studied. The walls are electrically conducting, which implies strong currents flowing within the thin conducting walls. The problem is solved both analytically and numerically.It is shown that the Joule heat cannot be neglected in certain range of parameters relevant to fusion blanket applications. The magnitude of the Joule heat released inside the channel and the walls depends on the thermal conductivity of the outside surface of the channel walls. For thermally conducting outside surface of the walls the Joule heat can become significant for high values of the Hartmann number and moderate average velocity. The effect is even more pronounced for thermally insulating outside surface of the walls. For example, for lead–lithium flow with stainless steel walls the temperature increase along the flow exceeds 200 °C over the length of the blanket, which is almost three times higher than that for thermally conducting outside surface of the walls.The main reason for such a strong rise in temperature is the heat released inside the walls. The heat produced in the fluid region is quickly convected towards the exit from the channel. The heat released inside the walls can only leave the domain by diffusion into the fluid region and thus is accumulated along the channel length.     

Natural Convection Heat Transfer in an Inclined Enclosure Filled with a Water-Cuo Nanofluid

This article presents the results of a numerical study on natural convection heat transfer in an inclined enclosure filled with a water-CuO nanofluid. Two opposite walls of the enclosure are insulated and the other two walls are kept at different temperatures. The transport equations for a Newtonian fluid are solved numerically with a finite volume approach using the SIMPLE algorithm. The influence of pertinent parameters such as Rayleigh number, inclination angle, and solid volume fraction on the heat transfer characteristics of natural convection is studied. The results indicate that adding nanoparticles into pure water improves its heat transfer performance; however, there is an optimum solid volume fraction which maximises the heat transfer rate. The results also show that the inclination angle has a significant impact on the flow and temperature fields and the heat transfer performance at high Rayleigh numbers. In fact, the heat transfer rate is maximised at a specific inclination angle depending on Rayleigh number and solid volume fraction.     

Laminar conjugate mixed convection in a vertical channel with heat generating components

Conjugate mixed convection arising from protruding heat generating ribs attached to substrates (printed circuit boards) forming channel walls is numerically studied. The substrates with ribs form a series of vertical parallel plate channels. Assuming identical disposition and heat generation of the ribs on each board, a channel with periodic boundary conditions in the transverse direction is considered for analysis. The governing equations are discretised using a control volume approach on a staggered mesh and a pressure correction method is employed for the pressure–velocity coupling. The solid regions are considered as fluid regions with infinite viscosity and the thermal coupling between the solid and fluid regions is taken into account by the harmonic thermal conductivity method. Parametric studies are performed by varying the heat generation based Grashof number in the range 10⁴–10⁷ and the fan velocity based Reynolds number in the range 0–1500, with air as the working medium. Results are obtained for the velocity and temperature distributions, natural convection induced mass flow rate through the channel, the maximum temperatures in the heat sources and the local Nusselt numbers. The natural convection induced mass flow rate in mixed convection is correlated in terms of the Grashof and Reynolds numbers. In pure natural convection the induced mass flow rate varies as 0.44 power of Grashof number. The maximum dimensionless temperature is correlated in terms of pure natural convection and forced convection inlet velocity asymptotes. For the parameter values considered, the heat transferred to the working fluid via substrate heat conduction is found to account for 41–47% of the heat removal from the ribs.     

Numerical Investigation on Thermal Performance Design of Cryogenic Compact Heat Exchangers with Serrated-Fin Channels

Abstract

Traditional empirical formulas of Colburn heat transfer factors will lead to a design deviation for cryogenic heat exchangers. This paper employs the computational fluid dynamics (CFD) technique to numerically study the thermal performance of cryogenic compact heat exchangers (CCHEs). To obtain more precise convective heat transfer coefficients, the heat transfer performance of CCHE with serrated fin channels is analyzed considering various cryogenic fluid properties, fin materials and the axial heat conduction (AHC), and a heat transfer deterioration rate is proposed to investigate the effect of AHC on the heat transfer performance of CCHEs. For the simulation design, a quasi-one-dimensional calculation model is developed to obtain the temperature and pressure fields of the whole heat exchanger using the previous CFD results of the finned channels to avoid the deviation caused by traditional empirical formulas. Finally, a case study for a CCHE in a practical system is designed and analyzed by the proposed approach. The results suggest that cryogenic conditions have a significant effect on the design performance of heat exchangers, especially when considering the influences of fluid properties, materials, and AHC. For different cryogenic fluids, accurate heat transfer factors should be selected for the design calculations, and materials with high thermal conductivity will increase the effect of AHC and deteriorate the performance of the CCHE.     

Transient modeling of micro-grooved heat pipe

One-dimensional transient model for fluid flow and heat transfer is presented for a micro-grooved heat pipe of any polygonal shape utilizing a macroscopic approach. The coupled non-linear governing equations for the fluid flow, heat and mass transfer are developed based on first principles and are solved simultaneously. The transient behavior for various parameters, e.g. substrate temperature, radius of curvature, liquid velocity, etc. are studied. The effects of the groove dimensions, heat input and Q-profiles on the studied parameters have been evaluated. The steady state profiles for substrate temperature, radius of curvature, liquid velocity etc. have also been generated. The model predicted steady state substrate temperature profile is successfully compared with the experimental results from the previous study. The general nature of the model and the associated parametric study ensure the wide applicability of the model.     

A NUMERICAL STUDY OF NONISOTHERMAL REACTIVE FLOW IN A DUAL-SCALE POROUS MEDIUM UNDER PARTIAL SATURATION

ABSTRACT

This numerical study investigates the temperature and cure distribution during the flow of a reactive liquid in dual-scale fibrous porous media under partial saturation. An iterative, control-volume approach, based on energy and cure balances, is used for developing discretized equations in the channels and fiber tows of the two-layer model of a dual-scale porous medium. Significant differences in the average temperatures and cures within the channels and fiber tows are observed. The ratio of the channel and fiber-tow pore volumes, the ratio of liquid and fiber heat capacities, the fiber-bundle thermal conductivity, along with the reaction rate are identified as the important parameters for temperature and cure distributions.     

Thermal protection with liquid film in turbulent mixed convection channel flows

In this numerical study, a channel flow of turbulent mixed convection of heat and mass transfer with film evaporation has been conducted. The turbulent hot air flows downward of the vertical channel and is cooled by the laminar liquid film on both sides of the channel with thermally insulated walls. The effect of gas–liquid phase coupling, variable thermophysical properties and film vaporization are considered in the analysis. In the air stream, the k–ε turbulent model has been utilized to formulate the turbulent flow. Parameters used in this study are the mass flow rate of the liquid film B, Reynolds number Re, and the free stream temperature of the hot air T_o. Results show that the heat flux was dramatically increases due to the evaporation of liquid water film. The heat transfer increases as the mass flow rate of the liquid film decreases, while the Reynolds number and inlet temperature increase, and the influences of the Re and T_o are more significant than that of the liquid flow rate. It is also found that liquid film helps lowering the heat and mass transfer rate from the hot gas in the turbulent channel, especially at the downstream.     

Experimental Study of Heat Transfer and Pressure Loss in Channels with Miniature V Rib-Dimple Hybrid Structure

Abstract

In order to increase the thermal efficiency, the gas turbines are designed to operate at higher temperature, which requires highly efficient cooling structures for turbine blades. The dimples and ribs are effective surface structures to enhance the convective heat transfer in the gas turbine blade internal cooling. In the present study, a novel hybrid cooling structure with miniature V-shaped ribs and dimples is presented, and the heat transfer and pressure loss characteristics are obtained experimentally. The heat transfer performance of the rib–dimple structures, which include three different rib height-to-hydraulic diameter ratios of 0.017, 0.029 and 0.044 and one dimple configuration with the dimple depth-to-diameter ratio of 0.2, are studied by using the transient liquid crystal thermography technique for turbulent flow in rectangular channels within the Reynolds number range from 10,000 to 60,000. It is found that the miniature V-shaped ribs arranged upstream the dimples can significantly improve the heat transfer performance of the dimples, resulting in a more uniform heat transfer distribution on the surface. The V rib-dimple hybrid structure in the channel shows much higher heat transfer enhancement than the counterparts with only the dimples in the channels.     

Thermal Energy Storage Performance of Fatty Acids as a Phase Change Material

Thermal energy storage performance of fatty acids and a eutectic mixture as phase change materials (PCMs) has been investigated experimentally. The selected PCMs for this study were palmitic acid, myristic acid, stearic acid, and a mixture of stearic and myristic acids in eutectic combination ratio of 65.7 wt% myristic acid and 34.3 wt% stearic acid. The PCMs have a melting temperature range of 50.0°C to 61.20°C and a latent heat range of 162.0 J/g to 204.5 J/g. The inlet temperature and the mass flow rate of heat transfer fluid (HTF) were selected as experimental parameters to test the thermal energy storage performance of the PCMs. The transition times, temperature range, propagation of the solid-liquid interface, as well as heat flow rate characteristics of the employed cylindrical tube storage system were studied at varied experimental parameters. The experimental results show that the melting front moves to inward in the radial directions as well as in the axial directions from the top toward to the bottom of the PCM tube. It was observed that the convection heat transfer in the liquid phase plays an important role in the melting process. The changes in the studied HTF parameters have more effect on the melting processes than the solidification processes of the PCMs. The average heat storage efficiency calculated from data for all the PCMs is 51.5%, meaning that 48.5% of the heat actually was lost somewhere.