Heat Transfer Effectiveness of Three-Fluid Separated Heat Pipe Exchanger

A heat transfer model for three-fluid separated heat pipe exchanger was analyzed,and the temperature transfer matrix for general three-fluid separated heat exchanger working in parallel-flow or counter-flow mode was obtained.It was found that the forms of temperature transfer matrix are similar for heat pipe rows with equal or different heat transfer surface area.Furthermore,by using the temperature transfer matrix of the heat pipe exchanger,the relationship between heat transfer effectiveness θ 1,θ 2 and M,NTU,U,Δt i were derived for the exchanger operating in parallel-flow or counter-flow mode,and a simple special example was adopted to demonstrate the correctness of these relationships.

Mixed convective heat and mass transfer analysis for peristaltic transport in an asymmetric channel with Soret and Dufour effects

The present investigation addresses the simultaneous effects of heat and mass transfer in the mixed convection peristaltic flow of viscous fluid in an asymmetric channel. The channel walls exhibit the convective boundary conditions. In addition, the effects due to Soret and Dufour are taken into consideration. Resulting problems are solved for the series solutions. Numerical values of heat and mass transfer rates are displayed and studied. Results indicate that the concentration and temperature of the fluid increase whereas the mass transfer rate at the wall decreases with increase of the mass transfer Biot number. Furthermore, it is observed that the temperature decreases with the increase of the heat transfer Biot number.

Heat transfer enhancement in MOSFET mounted on different FR4 substrates by thermal transient measurement

Miniaturization of electronic package leads to high heat density and heat accumulation in electronics device, resulting in short life time and premature failure of the device. Junction temperature and thermal resistance are the critical parameters that determine the thermal management and reliability in electronics cooling. Metal oxide field effect transistor（MOSFET）is an important semiconductor device for light emitting diode-integrated circuit（LED IC） driver application, and thermal management in MOSFET is a major challenge. In this study, investigations on thermal performance of MOSFET are performed for evaluating the junction temperature and thermal resistance. Suitable modifications in FR4 substrates are proposed by introducing thermal vias and copper layer coating to improve the thermal performance of MOSFET. Experiments are conducted using thermal transient tester（T3ster） at 2.0 A input current and ambient temperature varying from25℃ to 75℃. The thermal parameters are measured for three proposed designs： FR4 with circular thermal vias, FR4 with single strip of copper layer and embedded vias, and FR4 with I-shaped copper layer, and compared with that of plain FR4 substrate. From the experimental results, FR4I-shaped shows promising results by 33.71% reduction in junction temperature and 54.19% reduction in thermal resistance. For elevated temperature, the relative increases in junction temperature and thermal resistance are lower for FR4I-shaped than those for other substrates considered. The introduction of thermal vias and copper layer plays a significant role in thermal performance.

Numerical prediction of the long- term soil temperature variations around shallowsections of cross-river road tunnels

Considering the coupled heat transfer effect induced by parallel cross-river road tunnels, the long-term soil temperature variations of shallow sections of cross-river tunnels under the river beach are predicted using the finite difference method for numerical simulation. The boundary conditions and the initial values are determined by in situ observations and numerical iterations.The simulation results indicate that the ultimate calculated steady heat transfer time is 68 years, and most of the heat transfer is completed in 20 years.The initial constant temperature soil surrounding the tunnels is transformed to an annually variable one.An obvious temperature-varying region of the surrounding soil is discovered within 5 m from the tunnel exterior, as well as within the entire range of soil between the two tunnels.The maximum temperature increase value reaches 7.14 ℃ and the maximum peak-to-valley value of annual temperature increase reaches 10 ℃.The temperature variation of soils surrounding tunnels below 10 m is completely controlled by the heat transfer from the tunnels.The coupled heat transfer effect is confirmed because the ultimate steady temperature of soil between the two tunnels is higher than the ones along other positions.Moreover, the regression model comprising a series of univariate functions is proposed for the annual soil temperature fluctuation estimation for the locations varied distances around the tunnel.This investigation is beneficial to gain an insight into the long-term variation tendencies of local engineering geological conditions of the river beach above shallow sections of the cross-river road tunnels.

Effective thermal conductivity in granular media with devolatilization:the Lattice Boltzmann modelling

Flow thermomechanics in reactive porous media is of importance in industry including the thermal processing of fossil fuel(coking understood as a slow pyrolysis)involving devolatilisation.On the way to provide a detailed description of the process,a multi-scale approach was chosen to estimate effective transport coefficients.For this case the Lattice Boltzmann method(LBM)was used due to its advantages to accurately model multi-physics and chemistry in a random geometry of granular media.After account for earlier studies,the paper presents description of the model with improved boundary conditions and a benchmark case.Results from meso-scale LBM calculations are presented and discussed regarding the spatial resolution and the choice of relaxation parameter along its influence on the accuracy compared with empirical formulae.Regarding the estimation of effective thermal conductivity coefficient it is shown that occurrence of devolatilization has a crucial effect by reducing heat transfer.Some quantitative results characterise the propagation of thermal front;also presented is the evolution of effective thermal conductivity.The work is a step forward towards a physically sound simulation of thermal processing of fossil fuel.

Heat Transfer with Flow and Phase Change in an Evaporator of Miniature Flat Plate Capillary Pumped Loop

An overall two-dimensional numerical model of the miniature flat plate capillary pumped loop （CPL） evaporator is developed to describe the liquid and vapor flow, heat transfer and phase change in the porous wick structure, liquid flow and heat transfer in the compensation cavity and heat transfer in the vapor grooves and metallic wall. The entire evaporator is solved with SIMPLE algorithm as a conjugate problem. The effect of heat conduction of metallic side wall on the performance of miniature flat plate CPL evaporator is analyzed, and side wall effect heat transfer limit is introduced to estimate the performance of evaporator. The shape and location of vapor-liquid interface inside the wick are calculated and the influences of applied heat flux, liquid subcooling, wick material and metallic wall material on the evaporator performance are investigated in detail. The numerical results obtained are useful for the miniature flat plate evaporator performance optimization and design of CPL.

A Mixed Wavelet-Learning Method of Predicting Macroscopic Effective Heat Transfer Conductivities of Braided Composite Materials

In this paper,a novel mixed wavelet-learning method is developed for predicting macroscopic effective heat transfer conductivities of braided composite materials with heterogeneous thermal conductivity.This innovative methodology integrates respective superiorities of multi-scale modeling,wavelet transform and neural networks together.By the aid of asymptotic homogenization method(AHM),off-line multi-scalemodeling is accomplished for establishing thematerial databasewith highdimensional and highly-complexmappings.Themulti-scalematerial database and the wavelet-learning strategy ease the on-line training of neural networks,and enable us to efficiently build relatively simple networks that have an essentially increasing capacity and resisting noise for approximating the high-complexity mappings.Moreover,it should be emphasized that the wavelet-learning strategy can not only extract essential data characteristics from the material database,but also achieve a tremendous reduction in input data of neural networks.The numerical experiments performed using multiple 3D braided composite models verify the excellent performance of the presentedmixed approach.The numerical results demonstrate that themixedwaveletlearningmethodology is a robustmethod for computing themacroscopic effective heat transfer conductivities with distinct heterogeneity patterns.The presentedmethod can enormously decrease the computational time,and can be further expanded into estimating macroscopic effective mechanical properties of braided composites.

Temperature rise of He Ⅱ forced flow and its negative Joule-Thomson effect

The temperature rise of He Ⅱ transfer system due to the negative Joule-Thomson(JT)effect is one of the major problems in the He Ⅱ forced flow system design.Negative Joule-Thomson effect of the He Ⅱ forced flow was analyzed and calculated in this paper.The temperature rise due to the heat leak along the transfer pipeline was calculated by the simplified equation and was modified by considering the negative Joule-Thomson effect.The modified results were compared with the temperature rise obtained by non-linear differential equations with consideration of the pressure gradient.The results show that the pressure gradient has strong effect on the temperature distribution.The modified results are in good agreement with the values calculated by the complicated equation,which verifies the effectiveness of the simplified equation in calculating the temperature rise when the negative JT effect of He Ⅱ is known.