Showing papers on "Fin (extended surface) published in 2003"

Swirl flow heat transfer and pressure drop with twisted-tape inserts

Abstract: An extended review of the application of twisted-tape inserts in tubular heat exchangers and their thermal-hydraulic performance is presented. Twisted tapes promote enhanced heat transfer by generating swirl or secondary flows, increasing the flow velocity due to the tube partitioning and blockage, and providing an effectively longer helical flow length. Depending on the tape-edge to tube-wall contact, some fin effects may also be present. Their usage in both single-phase and two-phase (boiling and condensation) flows is considered, and heat transfer and pressure drop results from different investigations are presented. The characteristic features of swirl-induced heat transfer enhancement, nature of swirl flows and their scaling, and development of predictive correlations for heat transfer coefficients and friction factors (or pressure drop) are discussed. Also, some aspects of the use of geometrically modified twisted-tape inserts, as well as compound application with other enhancement techniques, are briefly discussed.

Epitaxially grown fin for FinFET

Abstract: A method of forming a fin for a fin field effect transistor (FinFET) includes defining a trench in a layer of first material, where a width of an opening of the trench is substantially smaller than a thickness of the layer. The method further includes growing a second material in the trench to form the fin and removing the layer of first material.

Exact solution for cooling of electronics using constructal theory

Abstract: This article deals with the constructal-theory based solution for conductive cooling of electronics. The problem falls in the category of a more general “area to point” flow problem. Heat generated in a fixed area is to be discharged to a heat sink located on the border of the heat generating area through relatively high conductive link(s). This will maintain a limited temperature difference between the hot spot inside the heat generating area and the heat sink. Constructal-theory based solution in this article is supported by and matched with an analytical and exact heat transfer analysis of the physical problem. The solution procedure starts with heat transfer analysis and geometric optimization of the smallest heat generating area. The assembly of optimized smallest areas in a fixed but larger heat generating area by introducing a high conductive link and geometric optimization of the area leads to achieve the goal of conductive cooling of a larger area. Sequence of assembly of optimized areas in a relatively larger area and geometric optimization of this area is continued until the required area size to be cooled is obtained. The process of assembly and optimization steps leads to the formation of a tree-network of high conductive links inside the heat generating area. Along with geometric optimization of a heat generating area in each step, the tree-network of high conductive links is optimized with respect to high conductive material allocation in the heat generating area as well.

Conjugate Heat Transfer Effects on a Realistic Film-Cooled Turbine Vane

Abstract: A conjugate heat transfer solver has been developed and applied to a realistic film-cooled turbine vane for a variety of blade materials. The solver used for the fluid convection part of the problem is the Glenn-HT general multiblock heat transfer code. The solid conduction module is based on the Boundary Element Method (BEM), and is coupled directly to the flow solver. A chief advantage of the BEM method is that no volumetric grid is required inside the solid – only the surface grid is needed. Since a surface grid is readily available from the fluid side of the problem, no additional gridding is required. This eliminates one of the most time consuming elements of the computation for complex geometries. Two conjugate solution examples are presented - a high thermal conductivity Inconel nickel-based alloy vane case and a low thermal conductivity silicon nitride ceramic vane case. The solutions from the conjugate analyses are compared with an adiabatic wall convection solution. It is found that the conjugate heat transfer cases generally have a lower outer wall temperature due to thermal conduction from the outer wall to the plenum. However, some locations of increased temperature are seen in the higher thermal conductivity Inconel vane case. This is a result of the fact that film cooling is a two-temperature problem, which causes the direction of heat flux at the wall to change over the outer surface. Three-dimensional heat conduction in the solid allows for conduction heat transfer along the vane wall in addition to conduction from outer to inner wall. These effects indicate that the conjugate heat transfer in a complicated geometry such as a film-cooled vane is not governed by simple one-dimensional conduction from the vane surface to the plenum surface, especially when the effects of coolant injection are included.

Mixed convection boundary-layer flow over a vertical surface embedded in a porous medium

Abstract: In this paper we have numerically investigated the existence and uniqueness of a vertically flowing fluid passed a model of a thin vertical fin in a saturated porous media. We have assumed the two-dimensional mixed convection from a fin, which is modelled as a fixed, semi-infinite vertical surface, embedded in a fluid-saturated porous media under the boundary-layer approximation. We have taken the temperature, in excess of the constant temperature in the ambient fluid on the fin, to vary as x λ , where x is measured from the leading edge of the plate and λ is a fixed constant. The Rayleigh number is assumed to be large so that the boundary-layer approximation may be made and the fluid velocity at the edge of the boundary-layer is assumed to vary as x λ . The problem then depends on two parameters, namely λ and e, the ratio of the Rayleigh to Peclet numbers. It is found that when λ>0 ( there are (is) dual (unique) solution(s) when e is grater than some negative values of e (which depends on λ). When λ 0 and λ

Effects of Varying Geometrical Parameters on Boiling From Microfabricated Enhanced Structures

Abstract: The current study involves two-phase cooling from enhanced structures whose dimensions have been changed systematically using microfabrication techniques. The aim is to optimise the dimensions to maximize the heat transfer. The entranced structure used in this study consists of a stacked network of interconnecting channels making it highly porous. The effect of varying the pore size, pitch and height on the boiling performance was studied, with fluorocarbon FC-72 as the working fluid. While most of the previous studies on the mechanism of enhanced nucleate boiling have focused on a small range of wall superheats (0-4 K), the present study covers a wider range (as high as 30 K) A larger pore and smaller pitch resulted in higher heat dissipation at all heat fluxes. The effect of stacking multiple layers showed a proportional increase in heat dissipation (with additional layers) in a certain range of wall superheat values only. In the wall superheat range 8-13 K, no appreciable difference was observed between a single layer structure and a three layer structure. A fin effect combined with change in the boiling phenomenon within the sub-surface layers is proposed to explain this effect.

Fan supplied heat exchanger fin performance under frosting conditions

Abstract: In this paper, a validated numerical model for frost growth on heat exchange fins is modified to simulate a fan-supplied finned heat exchanger under refrigeration frosting conditions. It is found that frost growth on refrigeration heat exchangers causes a dramatic drop in the fin heat rate, airflow rate, and fin efficiency while the pressure drop increases. A sensitivity study shows the effects of changing several design parameters including the type of fan.

A three dimensional numerical study on natural convection heat transfer from short horizontal rectangular fin array

Abstract: Steady state natural convection heat transfer in a longitudinally short rectangular fin array on a horizontal base is numerically investigated The problem is three dimensional laminar natural convection heat transfer with open boundaries A finite difference code based on vorticity-vector potential approach is developed to solve the governing equations The steady behaviour of the flow and temperature distributions is obtained from the solution of transient form of the governing equations Results are compared with the available experimental results in the literature A good agreement is seen between the numerical and reported experimental results Flow configurations occurring in the channel of the fin arrays with different geometrical parameters, (fin length, fin height and fin spacing) are analysed Based on the obtained results, two types of flow patterns are observed The mechanisms of the flows are discussed and flow patterns are plotted to support the discussion The study is limited to Rayleigh number based on fin spacing, ranging from 120 to 39000 The fin length and fin height are varied from 2 to 20 and 025 to 7-fin spacing, respectively

Constructal multi-scale structure for maximal heat transfer density

Abstract: This paper presents a new concept for generating the multi-scale structure of a finite-size flow system that has maximum heat transfer density–maximum heat transfer rate installed in a fixed volume. Laminar forced convection and parallel isothermal blades fill the volume. The spacings between adjacent blades of progressively smaller scales are optimized based on constructal theory: the goal is maximum heat transfer density. The smaller blades are installed in the fresh-fluid regions that sandwich the tips of the boundary layers of longer blades. The overall pressure difference is constrained. As the number of length scales increases, the flow rate decreases and the volume averaged heat transfer density increases. There exists a smallest (cutoff) length scale below which heat transfer surfaces are no longer lined by distinct (slender) boundary layers. Multi-scale flow structures for maximum heat transfer rate density can be developed in an analogous fashion for natural convection. The constructal multi-scale algorithms are deduced from principles, unlike in fractal geometry where algorithms are assumed.

Winglet-type vortex generators with common-flow-up configuration for fin-tube heat exchangers

Abstract: This investigation stems from the area of augmentation of heat transfer by generating streamwise longitudinal vortices. The vortex generators are arranged in a common-flow-up configuration. Existing air-cooled condensers in geothermal power plants use fin-tube heat exchangers with circular tubes. The heat exchangers are huge, and often the cost of the condensers is more than one-third of the plant cost. The size of the condensers can be reduced through enhancement of heat transfer from fin surfaces. The enhancement strategy involves introduction of strong swirling motion in the flow field. The swirl can be generated by the longitudinal vortices. In this study, the longitudinal vortices are created by delta winglet-type vortex generators, which are mounted behind the tubes. An element of a heat exchanger has been considered for detailed study of the flow structure and heat transfer analysis. Biswas and colleagues have obtained significant enhancement of heat transfer by deploying the winglet pair behind ea...

Aluminum alloy fin material for heat exchangers and heat exchanger including the fin material

Abstract: The present invention provides an aluminum alloy fin material for heat exchangers which has a thickness of 80 μm (0.08 mm) or less and excels in joinability to a tube material and in intergranular corrosion resistance. The aluminum alloy fin material is an aluminum alloy bare fin material or a brazing fin material which has a thickness of 80 μm or less and is incorporated into a heat exchanger made of an aluminum alloy manufactured by brazing through an Al—Si alloy filler metal. The structure of the core material before brazing is a fiber structure, and the crystal grain diameter of the structure after brazing is 50-250 μm. The Si concentration in the Si dissolution area on the surface of the fin material and at the center of the thickness of the fin material after brazing is preferably 0.8% or more and 0.7% or less, respectively.

Strategies for size reduction of microreactors by heat transfer enhancement effects

Abstract: One of the likely aims of reactor miniaturization in the field of chemical production and energy generation is to increase the conversion to the desired product and the selectivity of the process through better control of heat and mass transfer. In addition to the effects related to miniaturization, a further increase of the transfer coefficients is achieved by applying microstructuring techniques. In this context, three different approaches for heat transfer enhancement in miniaturized reaction systems are presented. The ideas put forward rely on entrance flow effects, inertial flows in meandering channels, and suppression of axial heat conduction. Among these ideas the entrance flow effect, realized by an arrangement of microfins with a typical dimension of a few hundred micrometers, provides the most efficient heat transfer. It is found that a heat transfer enhancement of at least one order of magnitude can be achieved compared to unstructured channels. On this basis, a miniaturized heat-exchanger reac...

Thermal Resistances of Circular Source on Finite Circular Cylinder With Side and End Cooling

Abstract: General solution for thermal spreading and system resistances of a circular source on a finite circular cylinder with uniform side and end cooling is presented The solution is applicable for a general axisymmetric heat flux distribution which reduces to three important distributions including isoflux and equivalent isothermal flux distributions The dimensionless system resistance depends on four dimensionless system parameters It is shown that several special cases presented by many researchers arise directly from the general solution Tabulated values and correlation equations are presented for several cases where the system resistance depends on one system parameter only When the cylinder sides are adiabatic, the system resistance is equal to the one-dimensional resistance plus the spreading resistance When the cylinder is very long and side cooling is small, the general relationship reduces to the case of an extended surface (pin fin) with end cooling and spreading resistance at the base The special case of an equivalent isothermal circular source on a very thin infinite circular disk is presented @DOI: 101115/11568124#

Vapor augmented heatsink with multi-wick structure

Abstract: A heat transfer device includes a base chamber, a fin chamber, and at least one fin. The chambers can be thermally coupled. The heat transfer device also includes a wick structure. The wick structure can include a multi-wick structure. The multi-wick structure can include a three-dimensional wick structure and/or a spatially varying wick structure.

Thermal design methodology for high-heat-flux single-phase and two-phase micro-channel heat sinks

Abstract: This paper explores several issues important to the thermal design of single-phase and two-phase micro-channel heat sinks. The first part of the paper concerns single-phase heat transfer in rectangular micro-channels. Experimental results are compared with predictions based on both numerical as well as fin analysis models. While the best agreement between predictions and experimental results was achieved with numerical simulation, a few of the fin models are found to provide fairly accurate predictions. The second part of the paper focuses on predicting the incipient boiling heat flux. A comprehensive model based on bubble departure and superheat criteria is developed and validated with experimental data. The incipience model is capable of predicting the location, shape and size of bubbles departing in rectangular micro-channels. In the third part of the study, an analytical model is developed to predict pressure drop across a two-phase micro-channel heat sink. This model provides a detailed assessment of pressure drop concerns with two-phase micro-channels, including compressibility, flashing and choking. Overall, the present study provides important guidelines concerning practical implementation of micro-channel heat sinks in high-heat-flux electronic cooling applications.

Experimental Investigation on Fluid Flow Maldistribution in Plate-Fin Heat Exchangers

Abstract: The plate-fin heat exchanger is normally designed with the assumption that the fluid is uniformly divided among all the parallel passages. In practice, however, the design of the exchanger, the heat transfer process, the operation of the external system, etc., may create high flow maldistribution. The performance deterioration of plate-fin heat exchangers due to flow maldistribution may be serious. In this review, the flow distribution performance in a plate-fin heat exchanger has been experimentally studied and the distribution performance of different distributors' inlet angles has been measured. The combined effects of the inlet angle and mass flow rate on flow maldistribution have been studied. The study is useful in the optimum design of plate-fin heat exchangers.

Potential benefits of smart refrigerant distributors. Final report.

Abstract: The main goal of this study was to investigate the benefits for finned tube refrigerant evaporators when refrigerant distribution was precisely controlled to produce a desired equal superheat in each circuit. This goal was accomplished by examining three different finned-tube evaporators; a wavy fin, wavy-lanced fin, and a wavy-lanced fin evaporator with tube sheets separated. The effects of non-uniform airflow on capacity were also examined while superheat was controlled in each evaporator circuit. In parallel with the experimental effort, a modelling program was implemented and validated with the experimental results and then used to determine the savings in evaporator core volume possible if refrigerant distribution was controlled by a smart distributor. In extreme cases, the savings in core volume could be as much as 40%. Extract from the table of contents; background and literature review; laboratory experiment (evaporators selected for testing; test conditions and experimental procedure; experimental results); evaporator modelling and simulations (heat transfer and pressure drop correlations; modelling issues: refrigerant distribution, internal heat transfer in a finned tube evaporator; possible savings in heat transfer area due to optimized superheat). The report can be downloaded from the Web site: www.arti-21cr.org/research/completed/index.html.

Heat transfer from extended surfaces subject to variable heat transfer coefficient

Abstract: The present article investigates the effect of locally variable heat transfer coefficient on the performance of extended surfaces (fins) subject to natural convection. Fins of different profiles have been investigated. The fin profiles presently considered are namely; straight and pin fin with rectangular (constant diameter), convex parabolic, triangular (conical) and concave parabolic profiles and radial fins with constant profile with different radius ratios. The local heat transfer coefficient was considered as function of the local temperature and has been obtained using the available correlations of natural convection for each pertinent extended surface considered. The performance of the fin has been expressed in terms of the fin efficiency. Comparisons between the present results for all fins considered and the results obtained for the corresponding fins subject to constant heat transfer coefficient along the fin are presented. Comparisons, i.e. showed an excellent agreement with the experimental results available in the literature. Results show that there is a considerable deviation between the fin efficiency calculated based on constant heat transfer coefficient and that calculated based on variable heat transfer coefficient and this deviation increases with the dimensionless parameter m.

Effects of fin shape on condensation in herringbone microfin tubes

Abstract: Effects of fin height and helix angle on condensation inside a herringbone microfin tube have been experimentally investigated with five types of herringbone microfin tubes. Heat transfer coefficients are about 2–4 times higher than that of the helical microfin tube under high mass velocity conditions. In the low mass velocity, they are equal to that of the helical microfin tube. The heat transfer enhancement increases with fin height up to 0.18 mm; higher fin heights show enhancement values similar to the 0.18 mm results. Pressure drop increases with the fin height. Larger helix angle yields higher heat transfer and higher pressure drop. For the lowest fin and/or smallest helix angle, the pressure drop is comparable with that of the helical microfin tube, while the heat transfer enhancement is higher. The enhancement mechanism is discussed from flow pattern observations. Effect of mass transfer resistance for R410A is estimated and negligible effects have been proved.

The Optimum Height of Winglet Vortex Generators Mounted on Three-Row Flat Tube Bank Fin

Abstract: Winglet vortex generators can be used to enhance the heat transfer performance of finned flat tube bank fin. The effects of the height of vortex generators (VG) on local heat transfer were studied using the naphthalene sublimation method and the optimum height of winglet VG are screened by using JF, a dimensionless factor of the larger the better characteristics. In order to get JF, the local heat transfer coefficient obtained in experiments and a numerical method were used to get the heat transferred from the fin. For the configurations studied in this paper: for local characteristic, as increasing height of VG, heat transfer is enhanced, but the mostly enhanced region moves away from the tube wall; with increasing height of VG to certain degree, the width of enhanced region does not increase significantly; the effects of VG's height on span-average Nusselt number (No) are more mixed on fin surface mounted with VGs and its hack surface, with increasing height of VG, in some region heat transfer is worsened, and in other region heat transfer is enhanced; in real working condition, the heat transferred from fin surface mounted with VGs is larger than the heat transferred from the other surface of the fin; increasing the height of VG (H) increases average Nu and friction factor (f), but with considering the fin efficiency, there is an optimum H to get best heat transfer performance; the optimum height of VG is dependent on the thickness of fin and its heat conductivity, for mostly used fin thickness and material, the optimum height of VG is 0.8 times of net fin spacing.

Heat dissipating device with uniform heat points

Abstract: A heat dissipating device with uniform heat points, having a first heat sink, a second heat sink and at least two heat pipes. The second heat sink is aligned over the first heat sink. Each heat pipe has a heat absorbing portion and a heat dissipating portion. The heat absorbing and dissipating portions of each heat pipe are in thermal communication with the first and second heat sinks, respectively. The distance between two neighboring heat absorbing portions is smaller than the distance between the heat pipe and the heat sink. Thereby, the heat absorbing portions are concentrated to absorb heat generated by a heat source, and the heat dissipating portions are distributed over a larger area to effectively dissipate the heat.