Showing papers in "Heat Transfer Engineering in 2012"

A Review of Metal Foam and Metal Matrix Composites for Heat Exchangers and Heat Sinks

Abstract: Recent advances in manufacturing methods open the possibility for broader use of metal foams and metal matrix composites (MMCs) for heat exchangers, and these materials can have tailored material properties. Metal foams in particular combine a number of interesting properties from a heat exchanger's point of view. In this paper, the material properties of metal foams and MMCs are surveyed, and the current state of the art is reviewed for heat exchanger applications. Four different applications are considered: liquid–liquid, liquid–gas, and gas–gas heat exchangers and heat sinks. Manufacturing and implementation issues are identified and discussed, and it is concluded that these materials hold promise both for heat exchangers and heat sinks, but that some key issues still need to be solved before broad-scale application is possible.

Recent Trends in Computation of Turbulent Jet Impingement Heat Transfer

Abstract: A review of the current status of computation of turbulent impinging jet heat transfer is presented. It starts with a brief introduction to flow and heat transfer characteristics of jet impinging flows considering the simplest jet impinging geometry: normal impingement of a single jet into a flat surface. Subsequently, a review of recent computational studies related to the same geometry is presented. The effects of different subgrid scale models, boundary conditions, numerical schemes, grid distribution, and size of the computational domain adopted in various large eddy simulations of this flow configuration are reviewed in detail. A review of direct numerical simulation of the same geometry is also presented. Further, some recent attempts in Reynolds-averaged Navier–Stokes modeling of impinging flows are also reviewed. A review of computation of other complex impinging flows is also presented. The review concludes with a listing of some important findings and future directions in the computation of impi...

Augmentation of Heat Transfer by Creation of Streamwise Longitudinal Vortices Using Vortex Generators

Abstract: This paper summarizes the current state of the art related to improvement of the heat exchanger surfaces using streamwise longitudinal vortices Primarily, the improvements related to fin-tube cross-flow heat exchangers and the plate-fin heat exchangers have been addressed Protrusions in certain forms, such as delta wings or winglet pairs, act as vortex generators, which can enhance the rate of heat transfer from the heat-exchanger surfaces that may be flat or louvered The strategically placed vortex generators create longitudinal vortices, which disrupt the growth of the thermal boundary layer, promote mixing between fluid layers, and hence lead to augmentation in heat transfer The flow fields are dominated by swirling motion associated with modest pressure penalty Heat transfer is augmented substantially for all the proposed configurations of the longitudinal vortex generators, such as delta wings, rectangular winglet pairs, and delta winglet pairs, with varying degree of pressure penalty Both comp

Polymer Heat Exchangers—History, Opportunities, and Challenges

Abstract: For the past 40 years considerable attention has been devoted to the innovation, characterization, and implementation of polymer heat exchanger technology, driven by the corrosion resistance, low density, low cost, and ease of manufacture of many polymeric materials. Moreover, new polymer composites, with higher impact and yield strengths, higher temperature limits, and higher thermal conductivities, promise to bridge the performance gap that exists between polymers and corrosion-resistant metals. This paper begins by reviewing the history of polymer heat exchangers and the technical limitations that have motivated much of the research on this technology. The notable developments that have taken place in the last decade and primary potential applications for polymer heat exchangers are then discussed, including solar water heaters, heat recovery systems, and seawater heat exchangers, in particular, for the desalination industry. The paper closes with a review of compact polymer heat exchangers, with milli...

Simultaneous Measurement of Thermal Conductivity, Thermal Diffusivity, and Specific Heat of Nanofluids

Abstract: Effective thermal conductivity, effective thermal diffusivity, and effective specific heat of nanofluids were simultaneously measured by using a transient double hot-wire technique. Several types of nanofluids were prepared by suspending different volume percentages (1 to 5%) of titanium dioxide (TiO2), aluminum oxide (Al2O3), and aluminum (Al) nanoparticles in ethylene glycol and engine oil. While effective specific heats of these nanofluids decrease substantially with nanoparticle volume fraction, the enhanced effective thermal conductivity and effective thermal diffusivity were found to increase significantly with increasing volumetric loading of these nanoparticles. The increments of the effective thermal diffusivity of nanofluids were slightly larger than their effective thermal conductivity values. Predictions of the effective specific heats of nanofluids by the volume fraction mixture rule-based model showed fairly good agreement (within 7%) with the experimental results. Besides particle volume fr...

Heat Transfer Properties of Nanodiamond–Engine Oil Nanofluid in Laminar Flow

Abstract: This study investigates the effects of utilizing nanodiamond as an additive to engine oil on the heat transfer enhancement in laminar pipe flow. A plain tube with internal diameter of 6 mm was used as the test section and heated by an electrical coil heater to produce a constant heat flux thermal boundary condition. Thermal conductivity, specific heat, and viscosity of nanofluids were measured for various volume fractions and temperatures. In addition, convection heat transfer coefficients and Nusselt numbers of nanofluids were obtained for different nanoparticle concentrations as well as various Peclet and Reynolds numbers. Experimental results clearly indicate the heat transfer enhancement due to the presence of nanoparticles in the fluid. However, pressure-drop measurements showed significant increase due to addition of nanopowder to engine oil.

Dropwise Condensation Studies on Multiple Scales

Abstract: Recent advances in nanotechnology, chemical/physical texturing and thin film coating technology generate definite possibilities for sustaining a dropwise mode of condensation for much longer durations than was previously possible. The availability of superior experimental techniques also leads to deeper understanding of the process parameters controlling the relevant transport phenomena, the distinguishing feature of which is the involvement of a hierarchy of length/time scales, proceeding from nuclei formation, to clusters, all the way to macroscopic droplet ensemble, drop coalescence, and subsequent dynamics. This paper is an attempt to connect and present a holistic framework of modeling and studying dropwise condensation at these multiple scales. After a review of the literature, discussions on the following problems are presented: (i) atomistic modeling of nucleation; (ii) droplet–substrate interaction; (iii) surface preparation; (iv) simulation of fluid motion inside sliding drops; (v) experimental ...

A Comparison of Metal-Foam Heat Exchangers to Compact Multilouver Designs for Air-Side Heat Transfer Applications

Abstract: High-porosity metal foams, with novel thermal, mechanical, electrical, and acoustic properties, are being more widely used in various industrial applications. In this paper, open-cell aluminum foam is considered as a highly compact replacement for conventional louver fins in brazed aluminum heat exchangers. A model based on the ϵ-NTU method is developed to compare the flat-tube, serpentine louver-fin heat exchanger to the flat-tube metal-foam heat exchanger. The two heat exchangers are subjected to identical thermal-hydraulic requirements, and volume, mass, and cost of the metal-foam and louver-fin designs are compared. The results show that the same performance is achieved using the metal-foam heat exchanger but a lighter and smaller heat exchanger is required. However, the cost of the metal-foam heat exchanger is currently much higher than that of the louver-fin heat exchanger, because of the high price of metal foams. If the price of metal foam falls to equal that of louver-fin stock (per unit mass), t...

A State-of-the-Art Review of Compact Vapor Compression Refrigeration Systems and Their Applications

Abstract: We present a critical review of the literature on the fundamentals, design, and application aspects of compact and miniature mechanical vapor compression refrigeration systems. Examples of such systems are those envisaged for electronics and personal cooling. In comparison to other refrigeration technologies (e.g., solid-state), vapor compression enables the attainment of low evaporating temperatures while maintaining a large cooling capacity per unit power input to the system. Over the past decade, there have been a significant number of studies devoted to the miniaturization of system components, with the most critical being the compressor. When compared with competing cooling technologies, such as flow boiling in microchannels, jet impingement, and spray cooling, refrigeration is the only one capable of lowering the junction temperature to values below the ambient temperature. The combination of vapor compression refrigeration with the aforementioned technologies is also possible, necessary, and benefi...

Loop Heat Pipes: A Review of Fundamentals, Operation,and Design

Abstract: The loop heat pipe (LHP) is a passive two-phase heat transport device that is gaining importance as a part of spacecraft thermal control systems and also in applications such as in avionics cooling and submarines. A major advantage of a loop heat pipe is that the porous wick structure is confuned to the evaporator section, and connection between the evaporator and condenser sections is by smooth tubes, thus minimizing pressure drop. A brief overview of loop heat pipes with respect to basic fundamentals, construction details, operating principles, and typical operating characteristics is presented in this paper. Finally, the paper presents the current developments in modeling of thermohydraulics and design methodologies of LHPs.

Foam Heat Exchangers: A Technology Assessment

Abstract: Open-cell porous metal foams have received attention for use in compact heat exchangers due to their increasing availability and improved thermal performance. In recent years, considerable research has been conducted on use of metallic and nonmetallic foams to further improve performance of state-of-the-art heat exchangers. In this paper, we report preliminary results from fabrication, development and experimental investigation of thermal-hydraulic performance of high-temperature metal foam heat exchangers for automotive exhaust gas recirculation (EGR) system. A brief review of nickel–chromium and stainless-steel foam heat exchanger technology and of recent efforts on their manufacturing techniques for a liquid-to-air heat exchange application is presented. Measured heat transfer and pressure drop data for foam heat exchangers and their comparison with performance of a conventional wavy plate-fin heat exchanger are discussed. Technical challenges and risks associated with foam heat exchangers are discusse...

Exergetic Optimization of Shell-and-Tube Heat Exchangers Using NSGA-II

Abstract: In this article, a multi-objective exergy-based optimization through a genetic algorithm method is conducted to study and improve the performance of shell-and-tube type heat recovery heat exchangers, by considering two key parameters, such as exergy efficiency and cost. The total cost includes the capital investment for equipment (heat exchanger surface area) and operating cost (energy expenditures related to pumping). The design parameters of this study are chosen as tube arrangement, tube diameters, tube pitch ratio, tube length, tube number, baffle spacing ratio, and baffle cut ratio. In addition, for optimal design of a shell-and-tube heat exchanger, the method and Bell–Delaware procedure are followed to estimate its pressure drop and heat transfer coefficient. A fast and elitist nondominated sorting genetic algorithm (NSGA-II) with continuous and discrete variables is applied to obtain maximum exergy efficiency with minimum exergy destruction and minimum total cost as two objective functions. The res...

Investigation of Thermal Conductivity and Viscosity of Carbon Nanotubes–Ethylene Glycol Nanofluids

Abstract: Conventional fluids used for heat transfer applications in automobiles limit the performance enhancement and compactness of the heat exchangers. These problems can be overcome by using the technology of nanofluids. The objectives of this work are to prepare nanofluids and to study their dynamic viscosity and thermal conductivity. Chemically treated carbon nanotubes (CNTs) were added with ethylene glycol (EG) and sonicated using a bath sonicator to have a homogeneous dispersion of CNTs in EG. In this study, the nanofluids were prepared with different concentrations of CNTs varying from 0.12 to 0.4 wt%. The dynamic viscosity of nanofluids was measured using a rheometer over a temperature range of 25°C to 60°C. It was observed that the viscosity of nanofluids decreases with an increase of temperature and enhances with CNT concentration. The nanofluid follows the characteristic behavior of Newtonian fluids. A linear rise in thermal conductivity of ethylene glycol was observed with an increase of CNT concentra...

Thermal Design of Multistream Plate Fin Heat Exchangers—A State-of-the-Art Review

Abstract: Multistream plate fin heat exchangers have replaced two-stream heat exchangers in diverse applications due to their compactness, capacity of handling multiple fluid streams in a single unit, and possibilities of having intermediate entry and exit of the streams. Unique features of such heat exchangers like direct/indirect crossover in temperatures due to several thermal communications among the fluid streams and the dependence of the thermal performance on “stacking pattern” have no equivalent in two-stream modules. As a consequence, an extension of the commonly used design/simulation techniques like ϵ-NTU or the LMTD method, applicable for two-stream exchangers, fails miserably in the case of multistream units. Though several techniques have been suggested over the years, in reality, no universally accepted methodology exists for the “thermal design” of multistream plate fin heat exchangers to date. In this communication, a state-of-the-art review of the thermal design of multistream plate fin heat excha...

Thermal Large Eddy Simulation in a Very Simplified Geometry of a Solar Receiver

Abstract: Thermal large eddy simulations are carried out in order to study the convective flow in a solar receiver. We investigate the impact of thermal gradients on a turbulent channel flow with imposed wall temperatures for two turbulent Reynolds numbers based on the friction velocity (180 and 395). In this configuration, the flow is subsonic, while temperature variations can be strong and induce significant variations of the fluid properties. The low Mach number equations are considered. The influence of the variations of the conductivity and the viscosity is first investigated. We show that the influence of these properties can be considered constant only for weak temperature gradients. The thermal subgrid-scale modeling is studied and we conclude that for a temperature ratio of 2, we can use a constant subgrid-scale Prandtl number. Finally, we focus on the increase of the temperature ratio that emphasizes the flow dissymmetry and modifies the fluctuations profiles. The physical mechanism responsible for these ...

Integrated Salts Precipitation and Nano-Filtration as Pretreatment of Multistage Flash Desalination System

Abstract: In this paper, the effect of salts precipitators (SP) and nano-filtration (NF) on the scale deposits of multistage flash (MSF) is studied using the Skillman index for estimating the likelihood of calcium sulfate scaling. The analysis was carried out to study the sulfate scale potential for seawater with 0 to 100% pretreated make-up in an MSF reference plant. The results showed that the scale potential increases with increasing temperature and decreases with increasing the percentage of either SP or NF-treated feed. For seawater with no feed pretreatment, the scale can start depositing at 115°C. However, the maximum top brine temperature (TBT), at which sulfate scale begins to precipitate, is shifted to higher temperatures with increased pretreated portion. For 100% SP-treated feed, TBT reached 170°C. The temperature is shifted to 120, 135, and 145°C when the NF-treated portion increases to 10, 25, and 50%, respectively. For 100% NF feed pretreatment, TBT can reach as high as 175°C.

Effect of Thermal Buoyancy on the Two-Dimensional Upward Flow and Heat Transfer Around a Square Cylinder

Abstract: The effect of aiding/opposing buoyancy on the two-dimensional upward flow and heat transfer around a heated/cooled cylinder of square cross section is studied in this work. The finite-volume-based commercial computational fluid dynamics (CFD) software FLUENT is used for the numerical simulation. The influence of aiding/opposing buoyancy is studied for Reynolds and Richardson numbers ranges of 50 to 150 and –1 to 1, respectively, and the blockage parameters of 2% and 25%. The flow exhibits unsteady periodic characteristics in the chosen range of Reynolds numbers (except for Reynolds number of 50 and blockage parameter of 25%) for the forced convective cases (Richardson number of 0). However, the vortex shedding is observed to stop completely at some critical value of Richardson number for a particular Reynolds number, below which the shedding of vortices into the stream is quite prominent. Representative streamlines and isotherm patterns for different blockage parameters are systematically presented and di...

Experimental Investigation on Flow Boiling Heat Transfer and Pressure Drop of HFC-134a inside a Vertical Helically Coiled Tube

Abstract: An investigation on flow boiling heat transfer and pressure drop of HFC-134a inside a vertical helically coiled concentric tube-in-tube heat exchanger has been experimentally carried out. The test section is a six-turn helically coiled tube with 5.786-m length, in which refrigerant HFC-134a flowing inside the inner tube is heated by the water flowing in the annulus. The diameter and the pitch of the coil are 305 mm and 45 mm, respectively. The outer diameter of the inner tube and its thickness are respectively 9.52 and 0.62 mm. The inner diameter of the outer tube is 29 mm. The average vapor qualities in test section were varied from 0.1 to 0.8. The tests were conducted with three different mass velocities of 112, 132, and 152 kg/m2-s. Analysis of obtained data showed that increasing of both the vapor qualities and the mass fluxes leads to higher heat transfer coefficients and pressure drops. Also, it was observed that the heat transfer coefficient is enhanced and also the pressure drop is increased when ...

Nucleate Boiling Enhancements on Porous Graphite and Microporous and Macro–Finned Copper Surfaces

Abstract: Enhancements in nucleate boiling heat removal with dielectric liquids, by increasing either the bubbles nucleation sites density and/or the wetted surface area, are desirable for immersion cooling of high-power computer chips. This article presents the results of recent investigations of nucleate boiling enhancement of FC-72, HFE-7100, and PF-5060 dielectric liquids on porous graphite, copper microporous surfaces, and copper surfaces with square corner pins, 3 mm × 3 mm in cross-section and 2, 3, and 5 mm tall. All surfaces have a footprint measuring 10 × 10 mm. These investigations examined the effects of liquid subcooling up to 30 K and surface inclination, from upward-facing to downward-facing, on nucleate boiling heat transfer coefficient and critical heat flux. Natural convection of dielectric liquids for cooling chips while in the stand-by mode, at a surface average heat flux <20 kW/m2, is also investigated for the different surfaces.

Effects of Structured Roughness on Fluid Flow at the Microscale Level

Abstract: It has been well established that there are no differences between microscale and macroscale flows of incompressible liquids. However, surface roughness has been known to impact the transport phenomena. This work aims to systematically quantify the effect of structured roughness geometries on friction factor in the laminar and turbulent flows as a precursor to the detailed heat transfer studies on these geometries. Experiments were conducted by varying the pitch (150–400 μm) and height (36–131 μm) of transverse rib roughness structures in rectangular channels such that the pitch-to-height ratio ranged from 2 to 8. The channel width was fixed at 12.70 mm and the length at 152.4 mm, while the channel gap was varied (230–937 μm). Tests were conducted over a Reynolds number range of 5–3400. The results are compared to the existing models, which do not account for specific roughness features such as pitch and height. A theoretical model is developed to predict the effect of roughness pitch and height on pressu...

Thermal Simulation of a Pulsating Heat Pipe: Effects of Different Liquid Properties on a Simple Geometry

Abstract: The pulsating heat pipe (PHP) is essentially a two-phase heat transfer device for low heat power applications (heat sinks, electronic cooling, etc.). Although it is a simple, cheap, and flexible structure, it is ruled by very complex physics, and a robust, validated simulation tool is still missing. In the present work the basic numerical model by Holley and Faghri (2005) has been updated with the latest fluid properties database and with the latest nondimensional heat transfer correlations in order to make it suitable for different working fluids. Good agreement between numerical results and experimental data coming from a single-loop PHP operating with ethanol is shown and, using a single “tuning” parameter, that is, the liquid film thickness around a vapor slug, which needs to be further experimentally investigated, the final goal of building a design tool for the PHP construction and implementation is getting closer.

Modeling of Air to Air Enthalpy Heat Exchanger

Abstract: The thermal performance of a Z-shaped enthalpy heat exchanger utilizing 45-gsm Kraft paper as the heat and moisture transfer surface for heating, ventilation, and air conditioning (HVAC) energy recovery is experimentally investigated through temperature and moisture content measurements. A mathematical model is developed and validated against the experimental results using the effectiveness-NTU method. In this model the paper moisture transfer resistance is determined by paper moisture permeability measurements. Results showed that the paper moisture transfer resistance is not constant and varies with moisture gradient across the paper. Furthermore, the model is used to predict the heat exchanger performance for different heat exchanger flow configurations. The results showed that higher effectiveness values are achieved when the heat exchanger flow path width is reduced. Temperature and moisture distribution in the heat exchanger is also studied using a computational fluid dynamics package (FLUENT). To m...

Preparation and Pool Boiling Characteristics of Silver Nanofluids Over a Flat Plate Heater

Abstract: Silver nanoparticles of 15 nm average size were prepared by sputtering method and were characterized through x-ray diffraction, atomic force microscopy, and transmission electron microscopy. The pool boiling heat transfer characteristics of nanofluids with 0.25%, 0.5%, and 0.75% by weight concentrations of silver nanoparticles suspended in distilled water are compared with that of pure water. Followed by the same concentration of silver nanoparticles in 9.0% weight of sodium lauryl sulfate anionic surfactant compared with that of pure water. The pool boiling heat transfer data were obtained for a 30-mm-square and 0.44-mm-thickness stainless-steel flat-plate heater. Results reveal that the silver nanoparticle enhances the heat transfer coefficient of water. The heat transfer coefficient increases with increase of silver nanoparticles concentration. The highest enhancement has been found to be 1.9 times greater than that of water at a heat flux of 120 kW/m2 with 0.75% of silver nanoparticles in water. In a ...

Heat transfer through vertically downward blowing single-jet air curtains for cold rooms

Abstract: One of the major sources of heat gain in refrigerated storage rooms is the infiltration of warm ambient air through doorways. Air curtains reduce this amount of heat transfer by blowing a plane air jet in the doorway while allowing an easy passage of the traffic. An air curtain device installed at the doorway of a cold room in a supermarket was studied in detail. Thermographic images were taken, recording the temperature field across the doorway. Tracer gas decay measurements were used to estimate the airflow rate through the door. These measurements were then used to validate a computational fluid dynamics (CFD) model of the air curtain. With this CFD model the impact of some important air curtain parameters, such as the jet velocity and the jet nozzle width, on the heat transfer rate through the opening is determined. Finally, an expression to estimate the heat transfer rate through the air curtain is proposed.

A Theory Describing Asphaltene Adhesion Fouling Inside Heat Exchanger Tubes

Abstract: A theory for mathematically modeling asphaltene adhesion fouling in heat exchanger tubes was derived and its agreement with experiment suggested feasibility. The premise of this theory is that asphaltene adhesion fouling requires the formation of a chemical bond—modeled here as a sulfur–sulfur bond—which is strong enough to resist fluid dynamic forces. This theory suggests that once an asphaltene monolayer is adsorbed onto a heat transfer surface, shear stress alone may be insufficient for preventing further fouling unless the asphaltene flocculate size distribution can be manipulated, the chemically labile heteroatoms can be deactivated, or the asphaltenes can be kept in solution. A method for calculating fouling threshold shear stresses and flocculate diameters is expounded.

Numerical Study of the Influence of Material Structure on Effective Thermal Conductivity of Concrete

Abstract: A two-dimensional numerical model is presented to examine meso- and macroscopic structure effects on the effective thermal conductivity of concrete. The heterogeneity of concrete is considered at a mesoscopic level by Weibull distribution assumption. Simulations on several heterogeneous samples show that the effective thermal conductivity strongly depends on the degree of heterogeneity. Higher homogeneity indicates a greater effect on the effective thermal conductivity. Numerically simulated results also indicate that the size and shape of individual coarse aggregate appear to have negligible influence on the effective thermal conductivity of concrete. However, that greatly depends on the thermal conductivity and volume fraction of coarse aggregate. Modeling suggests that heat conductivity decreases when there is a drop in strength due to the damages creating a thermal barrier across the cracks and thus preventing heat flow through the matrix, that is, resulting in reduction of effective thermal conductiv...

A Semi-Empirical Model for Predicting the Effective Dynamic Viscosity of Nanoparticle Suspensions

Abstract: A semi-empirical model for predicting the effective dynamic viscosity of nanofluids, based on a single set of experimental data available in the literature for silica nanoparticles suspended into ethanol, is presented and discussed. The equation, which includes the overall effects of the friction at the solid–liquid interface, the nanoparticle colloidal interactions, and the formation of aggregates, expresses the ratio between the effective dynamic viscosity of the suspension and the dynamic viscosity of the pure base liquid as a function of the nanoparticle size and concentration. The predicted results are in rather good agreement with a wide variety of data relative to nanofluids consisting of several combinations of solid and liquid phases, extracted from different sources. The accuracy and ease of application of the proposed equation make it interesting from the engineering point of view, for both numerical simulation purposes and thermal design tasks.

Experimental and Numerical Study of Recuperative Heat Recirculation

Abstract: In many power-generating systems heat recirculation is needed in order to increase their thermal efficiency and ensure sufficiently high temperatures required in fuel processing. The current article investigates experimentally and numerically a heat recirculation phenomenon by using gas–gas recuperation. The systematized description of practicalities of heat recirculation is reported. The experimental characteristics of a heat recirculator are presented in terms of the effect of power of the heater at constant mass flow rate, effect of mass flow rate at constant power of the heater, and effect of mass flow rate at constant ratio power of the heater/mass flow rate on heat transferred, heat lost to the surroundings, and processing temperatures. The results show that heat recirculation is maximized at moderate mass flow rate, at large power, and in miniaturized channels. Further, a numerical model is used in order to interpret and extend the experimental data set. The simulations are focused on heat recircul...

Numerical Study of Heat Transfer and Analysis of Optimal Fin Pitch in a Wavy Fin-and-Tube Heat Exchanger

Abstract: In this paper an analysis of laminar heat transfer and fluid flow in a wavy fin-and-tube heat exchanger has been carried out. Three-dimensional (3D) numerical simulation results of a circular tube heat exchanger were compared with published numerical and experimental results. The computational fluid dynamics (CFD) procedure was validated by comparing average Nusselt numbers, and good agreement between published and calculated results has been accomplished. The influence of inlet air velocity, varying from 0.5 to 5 m s−1, as well as fin pitch, varying from 0.4 to 4 mm, on heat transfer and pressure drop conditions has been studied. The results have shown that there is an optimal fin pitch for each air velocity, which gives the best heat exchanger performance from the heat transfer point of view.

Frictional and Heat Transfer Characteristics of Single-Phase Microchannel Liquid Flows

Abstract: In this paper, we review the literature on flow frictional and heat transfer characteristics of single-phase liquid flows through microchannels. The work accentuates the existing discord between experimental observations of microscale transport process characteristics and the corresponding theoretical predictions on the basis of the classical paradigms. The role of microscale effects in inducing such disparity between experimental and theoretical frameworks, as indicated by various researchers, is critically discussed. Theoretical models and empirical correlations, proposed in the literature, for comprehending microscale flow and thermal characteristics are also highlighted for ready reference. In closure, aspects of microscale liquid flow and heat transfer requiring further scrutiny are identified, and possible future research directions are prescribed.