Effects of fin shapes on heat transfer in microchannel heat sinks

In the present study, compact water cooling of high‐density, high‐speed, very‐large‐scale integrated (VLSI) circuits with the help of microchannel heat exchangers were investigated analytically. This study also presents the result of mathematical analysis based on the modified Bessel function of laminar fluid flow and heat transfer through combined conduction and convection in a microchannel heat sink with triangular extensions. The main purpose of this paper is to find the dimensions of a heat sink that give the least thermal resistance between the fluid and the heat sink, and the results are compared with that of rectangular fins. It is seen that the triangular heat sink requires less substrate material as compared to rectangular fins, and the heat transfer rate per unit volume has been almost doubled by using triangular heat sinks. It is also found that the effectiveness of the triangular fin is higher than that of the rectangular fin. Therefore, the triangular heat sink has the ability to dissipate large amounts of heat with relatively less temperature rise for the same fin volume. Alternatively, triangular heat sinks may thus be more cost effective to use for cooling ultra‐high speed VLSI circuits than rectangular heat sinks.     

Convective heat transfer in cross-corrugated triangular ducts under uniform heat flux boundary conditions

Cross-corrugated triangular ducts provide high heat mass transfer capabilities in membrane based air-to-air heat mass exchangers. The mixing effect would intensify the convective heat mass transfer coefficients on membrane surfaces. In this study, the fluid flow and convective heat transfer in a cross-corrugated triangular duct under uniform heat flux boundary condition is modeled and experimentally studied. A low Reynolds number k–ω (LKW) turbulence model is employed to account for the turbulence in the flow. Heat transfer experiments and high speed hot wire anemometry technology are used to validate the model. The transitional behavior of fluid flow in the duct is disclosed by velocity measurements and Fourier transforms. Correlations are provided for estimation of the pressure drop and the mean Nusselt numbers under uniform heat flux boundary conditions. The established correlations can be extended to estimate the convective mass transfer coefficients through heat mass analogy.     

Thermo-Hydraulic Performance Enhancement of Finned Elliptical Tube Heat Exchangers by Utilizing Innovative Dimple Turbulators

Abstract

As a new type of fin structure in finned tube heat exchangers, dimple turbulators exhibit excellent potential for thermo-hydraulic performance enhancement. A three-dimensional numerical simulation study was conducted to investigate the influences of five kinds of innovative concave dimple turbulators (CDTs), namely – elliptical dimple, conical frustum dimple, trapezoidal prism dimple, leeward triangular dimple and upward triangular dimple (UwTD) on the thermo-hydraulic performance enhancement in a plate fin-and-elliptical tube (PFET) heat exchanger, where CDTs are textured on the fin surface transversely between the elliptical tubes. The computational results are analyzed by considering the performance evaluation criterion for the PFET heat exchangers with different types of CDT shapes. The present investigation demonstrates that the heat transfer enhancement is intimately pertained to ejection with longitudinal counter-rotating flow, strengthened secondary flow and vortex structures at the downstream rim of CDT. A parametric study on the CDTs indicated that the UwTD vortex turbulators give better thermo-hydraulic performance under the present conditions. The numerical simulation results illustrated different secondary flow structures and heat transfer characteristics of the CDTs with various shapes, which disclosed the influential mechanisms of differently shaped dimple turbulators on the heat transfer augmentation in PFET heat exchangers.     

THERMOSOLUTAL TRANSFER WITHIN TRAPEZOIDAL CAVITY

The heat and mass transfer problem in a trapezoidal cavity is treated in this article. The lower part of the cavity is heated and the top inclined part is cooled. Phenomenological equations are solved using the alternating direction implicit (ADI) method combined with a fourth-order compact Hermitian method. The results are compared to those obtained experimentally and numerically by other authors in the triangular and trapezoidal cavity cases. The thermoconvective instabilities obtained are similar to those obtained in rectangular cavities. The influence of geometric parameters, global solicitations, and Lewis numbers on fluid flow configurations and on heat and mass transfer ratios is also studied.     

Enhancement of convective heat transfer in an air-cooled heat exchanger using interdigitated impeller blades

The enhancement of convective heat transfer through a finned heat sink using interdigitated impeller blades is presented. The experimentally investigated heat sink is a subcomponent of an unconventional heat exchanger with an integrated fan, designed to meet the challenges of thermal management in compact electronic systems. The close integration of impeller blades with heat transfer surfaces results in a decreased thermal resistance per unit pumping power. The performance of the parallel plate air-cooled heat sink was experimentally characterized and empirically modeled in terms of nondimensional parameters. Dimensionless heat fluxes as high as 48 were measured, which was shown to be about twice the heat transfer rate of a traditional heat sink design using pressure-driven air flow at the same mass flow rate.     

螺旋扭曲椭圆管换热器壳程数值模拟

以水为介质,采用k-ε模型,用数值模拟方法研究了5种不同结构的螺旋扭曲椭圆管换热器的管外壳程传热与流阻性能,并和采用椭圆管作为换热部件的换热器进行了比较.研究结果表明,螺旋扭曲椭圆管换热器壳程有较好的强化换热特性,螺旋扭曲椭圆管的几何尺寸和流体流动速度对壳程传热与流阻性能有重要影响.通过数值模拟所获得的规律为螺旋扭曲椭...     

Experimental investigation into the thermal-flow performance characteristics of an evaporative cooler

This study investigates the thermal-flow performance characteristics of an evaporative cooler. The derivation of the Poppe [1] and Merkel [2] analysis for evaporative coolers are presented and discussed. Performance tests were conducted on an evaporative cooler consisting of 15 tube rows with 38.1 mm outer diameter galvanized steel tubes arranged in a 76.2 mm triangular pattern. From the experimental results, correlations for the water film heat transfer coefficient, air–water mass transfer coefficient and air-side pressure drop are developed. The experimental tests show that the water film heat transfer coefficient is a function of the air mass velocity, deluge water mass velocity as well as the deluge water temperature, while the air–water mass transfer coefficient is a function of the air mass velocity and the deluge water mass velocity. It was found that the correlations obtained for the water film heat transfer coefficient and the air–water mass transfer coefficient compare well with the correlations given by Mizushina et al. [3]. Relatively little published information is available for predicting the air-side pressure drop across deluged tube bundles. The present study shows that the pressure drop across the bundle is a function of the air mass velocity and the deluge water mass velocity.     

双绝热圆柱对水平三角腔内自然对流影响的数值研究

针对含双绝热圆柱的底部加热水平等腰三角腔内空气的稳态层流自然对流开展研究.通过有限容积法对控制方程进行了数值求解,其中瑞利数的变化范围为104 ～106,圆柱体的尺寸比则分别为0（无圆柱体）、1/8和1/4.基于计算结果对自然对流的流动与传热特性随瑞利数和尺寸比的变化规律进行了分析和讨论.结果表明,双绝热圆柱的存在较大程度上改变了三角腔内自然对流的流型和温度场分布,但对整体传热影响较小,仅略微提高了平均努赛尔数,强化传热的效果在尺寸比为1/8时较为明显.     

Heat Transfer in a Loop Heat Pipe using Diamond-H2O Nanofluid

The main aim of this study is to enhance the thermal performance of loop heat pipe (LHP) charged with nanofluid as the working fluid. Thus, experiments are conducted to investigate heat transfer characteristics of using diamond-H₂O nanofluid with nanoparticle mass concentration ranged from 0% to 3% in a LHP as a working medium for heat input range from 20 W to 60 W. The three-dimensional model, laminar flow and heat transfer governing equations are solved using the finite volume method. The simulations are carried out with three-dimensional model based on the characterization of the working fluid inside the LHP to give an insight into the heat transfer and fluid flow mechanism. The LHP performance is evaluated in terms of temperature distributions and total thermal resistance of LHP. It is inferred that the temperatures obtained at all points in evaporator side of LHP charged with diamond-H₂O nanofluid are lower and reach their steady state faster than LHP charged with pure water. At the constant heat input, test results showed the average decrease of 5.7%?10.8% at nanoparticle mass concentrations ranging from 0.5% to 3% in R_th of LHP as compared with pure water (0%).     

Heat Transfer and Pressure Loss in Combined Plain and Diagonally Finned Heating Surfaces

This paper presents the results of model investigation of the heat transfer in plain and combined plain and diagonally finned convective tube banks. A heat and mass transfer analogy, by means of a naphthalene sublimation technique, is used. The effect of tube bank arrangement on heat transfer coefficients and flow resistance is discussed. The results show an increase of Nusselt numbers in comparison to those obtained for plain tube arrangements; however, a significantly higher flow resistance accompanies this increase.     

Heat transfer and pressure drop in spacer-filled channels for membrane energy recovery ventilators

This article investigates various support spacers for airflow through membrane-bound channels in energy recovery ventilators (ERVs) to enhance heat and mass transfer. Although liquid flow through membrane-bound channels has been extensively investigated, little work has looked at airflow through these channels. This article presents theoretical pressure drop and heat transfer for an open channel and for simple triangular corrugation (or plain-fin) spacers, which are common in heat exchangers and in some ERVs. It then presents the experimental pressure drop and heat transfer for two new corrugated mesh spacers, with one spacer in three orientations. Results indicate that these can improve heat transfer with little pressure-drop penalty compared to the triangular corrugation spacers. Results also show that unsteady flow occurs in the mesh spacers once a certain flow rate is reached. The optimal spacer depends on the application, which is shown with a cost savings estimate for a hypothetical ERV. Simpler performance metrics that do not require cost estimates can be used to compare two spacers, as long as their limitations are considered.     

Flow field and heat transfer analysis of local structure for regenerative cooling panel

Local structure of cooling panel has great effects on the heat transfer in the cooling channel for the scramjet. The problems of flow dead area and mass flow rate non-uniform distribution caused by the local structure effect the cooling effectiveness in the channel seriously. Numerical simulation to the flow field of scramjet cold panel with four different fuel injection island structures respectively has been carried out using the CFD commercial software-CFX in this research. The results reveal that flow dead area has been eliminated and flow field has been improved for the optimized structure. Furthermore, local resistance loss has been decreased and the mass flow rate non-uniform distribution in the channel has been reduced. Based on the optimized results, some suggestions about the local design of cooling panel have been proposed in this research.     

NUMERICAL PREDICTION OF LAMINAR FORCED CONVECTION IN TRIANGULAR DUCTS WITH UNSTRUCTURED TRIANGULAR GRID METHOD

The flow and heat transfer characteristics of smooth triangular ducts with different apex angles of 15, 30, 60, and 90 under the fully developed laminar flow condition were predicted numerically using a finite volume method. The SIMPLE-like algorithm was employed together with an unstructured triangular grid method, where the grid was generated by a Delaunay method. The triangular grid was adopted instead of the traditional rectangular grid to fit better into the triangular cross section of the duct. Two kinds of boundary condition (uniform wall temperature and uniform wall heat flux) were considered. Comparison of the predictions with previous computational results indicated a very good agreement. Both the friction factor and Nusselt number (Nu) showed a strong dependence on apex angle of the triangular duct. When the apex angle was 60, the duct provided the highest steady-state forced convection from its inner surface to the airflow under the laminar flow condition.     

Comparison of parallel and counter flow coil absorber performance

This study concentrates on the absorber used in the vapor absorption systems using water–lithium bromide solution with water as the refrigerant and investigates the simultaneously occurred heat and mass transfer during the absorption process. The heat and mass transfer equations were applied to simulate this process and solved using a computer program written in Delphi 7 for the parallel and counter flow absorbers. The simulation results were compared with the results of the past studies. The solution and cooling water temperatures, the overall heat transfer coefficient, the heat transferred and the mass absorbed were calculated for the parallel and counter flow absorbers. It is concluded that the counter flow absorber presents better performance for all conditions. For smaller number of coils, the difference is smaller, however if the number of coils is bigger, the counter flow absorber presents much better performance than the parallel flow absorber. When the number of coils is 20 and 120, the counter flow absorber provides 1.7% and 26% higher heat and mass transfer than the parallel flow absorber respectively.     

Theoretical analysis of film condensation heat transfer inside vertical mini triangular channels

An analytical model is presented for predicting film condensation of vapor flowing inside a vertical mini triangular channel. The concurrent liquid-vapor two-phase flow field is divided into three zones: the thin liquid film flow on the sidewall, the condensate flow in the corners, and the vapor core flow in the center. The model takes into account the effects of capillary force induced by the free liquid film curvature variation, interfacial shear stress, interfacial thermal resistance, gravity, axial pressure gradient, and saturation temperatures. The axial variation of the cross-sectional average heat transfer coefficient of steam condensing inside an equilateral triangular channel is found to be substantially higher than that inside a round tube having the same hydraulic diameter, in particular in the entry region. This enhancement is attributed to the extremely thin liquid film on the sidewall that results from the liquid flow toward the channel corners due to surface tension. The influences of the inlet vapor flow rates, the inlet subcooling, and the channel size on the heat transfer coefficients are also examined.     

Finite element simulation of mixed convection heat and mass transfer in a right triangular enclosure

A simulation of mixed convection heat and mass transfer in a right triangular enclosure is investigated numerically. The bottom surface of the enclosure is maintained at uniform temperature and concentration that are higher than that of the inclined surface. Moreover, the left wall of cavity moves upward (case 1) and downward (case 2) directions, which have constant flow speed, and is kept adiabatic. The enclosure represents the most common technology utilizing solar energy for desalination or waste-water treatment. A simple transformation is employed to transfer the governing equations into a dimensionless form. A finite-element scheme is used for present analysis. Comparison with the previously published work is made and found to be an excellent agreement. The study is performed for pertinent parameters such as buoyancy ratio, Richardson number and the direction of the sliding wall motion. The effect of aforesaid parameters on the flow and temperature fields as well as the heat and mass transfer rate examined. The results show that the increase of buoyancy ratio enhances the heat and mass transfer rate for all values of Richardson number and for each direction of the sliding wall motion. However, the direction of the sliding wall motion can be a good control parameter for the flow and temperature fields.     

Modeling of double-layer triangular microchannel heat sink based on thermal resistance network and multivariate structural optimization using firefly algorithm

Abstract

The micro-channel heat dissipation system has minor specifications and good thermal conductivity per unit, which is the best choice for heat dissipation of micro-chips. By optimizing the cross section of microchannel, the heat exchange efficiency and temperature uniformity can be effectively improved. In this article, a double-layer triangular microchannel heat sink is proposed, which uniquely combines triangular cross section and double-layer structure to obtain a better heat dissipation performance. A new thermal resistance network model is established. At the same time, the model of pressure drop in microchannel heat sink is obtained by use of fluid theory. Taking thermal resistance and pressure drop as optimization objectives, the thermal resistance of double-layer triangular microchannel heat sink is 0.284?K/W and the pressure drop is 1386.89?Pa by using the firefly algorithm based on the Pareto optimal solution set, obtaining the optimal structural parameters. The thermal-flow-solid coupling simulation analysis shows that the thermal resistance and theoretical analysis error is 5.19%, and the pressure drop and theoretical analysis error is 9.49%, which can verify the accuracy of the thermal resistance network model. This article has a guiding significance for the thermal resistance analysis and heat dissipation improvement of non-rectangular cross section microchannel heat sinks.     

Characterization of heat transfer between phases inside a porous medium as applied to vegetal set representations

Convective heat transfer between vegetal sets and the surrounding air in the context of forest fires has not yet been fully investigated and understood in existing studies. This process may have a great influence on many environmental problems such as forest fires. This study is devoted to the computational heat transfer characterization of tree structures. These structures were generated by Iterated Function Systems (IFS) and the fluid flow was computed using balance equations (mass, momentum, heat, etc.). The heat transfer was then characterized using the macroscopic Stanton number on several tree structures. The main objective of this study was to demonstrate that the macroscopic Stanton number only depends on the macroscopic Reynolds number using a power law. In addition, the power law exponent was found to be quasi-constant for all the configurations tested in this work and it tends to be universal.     

Experimental and theoretical study on single-phase natural circulation flow and heat transfer under rolling motion condition

Experimental and theoretical studies of single-phase natural circulation flow and heat transfer under a rolling motion condition are performed. Experiments with and without rolling motions are conducted so the effects of rolling motion on natural circulation flow and heat transfer are obtained. The experimental results show the additional inertia caused by rolling motion easily causes the natural circulation flow to fluctuate. The average mass flow rate of natural circulation decreases with increases in rolling amplitude and frequency. Rolling motion enhances the heat transfer, and the heat transfer coefficient of natural circulation flow increases with the rolling amplitude and frequency. An empirical equation for the heat transfer coefficient under rolling motion is achieved, and a mathematical model is also developed to calculate the natural circulation flow under a rolling motion condition. The calculated results agree well with experimental data. Effects of the rolling motion on natural circulation flow are analyzed using the model. The increase in the flow resistance coefficient is the main reason why the natural circulation capacity decreases under a rolling motion condition.     

Forced convective boiling of ternary mixtures at high qualities

This paper deals with convective boiling of ternary mixtures in vertical tubes. Experiments were carried out in 8.58 m long, 25.4 mm ID, electrically heated test section using an n-pentane/n-hexane/iso-octane mixture (0.31/0.22/0.47 overall mole fraction). Bulk and wall temperatures as well as local heat transfer coefficients were measured. The results were compared with those obtained using an extension to ternary (and multicomponent) mixtures of the methodology proposed by J.R. Barbosa and G.F. Hewitt [Int. J. Heat Mass Transfer 44 (2001) 1465, 1475] for prediction of forced convective boiling of binary mixtures in upward annular flow. Interphase transfer of heat and mass is dealt with using two formulations: (i) a film method in which the diffusive fluxes are calculated via a linearly generalised Fick's Law [H.L. Toor, AIChE J. 10 (4) (1964) 460; W.E. Stewart, R. Prober, Ind. Eng. Chem. Fundam. 3 (3) (1964) 224], and (ii) an effective diffusivity method. Droplet entrainment and deposition are modelled via an interchange model that takes into account the local concentration non-equilibrium between the liquid film and the entrained droplets. As for the binary case, it is claimed that the deterioration in the heat transfer coefficient is due to a combined effect of droplet interchange and mass transfer resistance in the vapour side.