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1.
An analysis of a crossflow plate-fin compact heat exchanger, accounting for the combined effect of two-dimensional longitudinal
heat conduction through the exchanger wall and nonuniform inlet fluid flow distribution on both hot and cold fluid sides is
carried out using a finite element method. Using the fluid flow maldistribution models, the exchanger effectiveness and its
deterioration due to the combined effects of longitudinal heat conduction and flow nonuniformity are calculated for various
design and operating conditions of the exchanger. It was found that the performance deteriorations are quite significant in
some typical applications due to the combined effects of wall longitudinal heat conduction and inlet fluid flow nonuniformity
on crossflow plate-fin heat exchanger.
Received on 20 May 1999 相似文献
2.
This paper presents a numerical method for determining heat transfer coefficients in cross-flow heat exchangers with extended heat exchange surfaces. Coefficients in the correlations defining heat transfer on the liquid- and air-side were determined using a non-linear regression method. Correlation coefficients were determined from the condition that the sum of squared fluid temperature differences at the heat exchanger outlet, obtained by measurements and those calculated, achieved minimum. Minimum of the sum of the squares was found using the Levenberg-Marquardt method. The outlet temperature of the fluid leaving the heat exchanger was calculated using the mathematical model describing the heat transfer in the heat exchanger. Since the conditions at the liquid-side and those at the air-side are identified simultaneously, the derived correlations are valid in a wide range of flow rate changes of the air and liquid. This is especially important for partial loads of the exchanger, when the heat transfer rate is lower than the nominal load. The correlation for the average heat transfer coefficient on the air-side based on the experimental data was compared with the correlation obtained from numerical simulation of 3D fluid and heat flow, performed by means of the commercially available CFD code. The numerical predictions are in good agreement with the experimental data. 相似文献
3.
This paper presents a new predictive model of droplet flow and heat transfer from molten salt droplets in a direct contact heat exchanger. The process is designed to recover heat from molten CuCl in a thermochemical copper–chlorine (Cu–Cl) cycle of hydrogen production. This heat recovery occurs through the physical interaction between high temperature CuCl droplets and air. Convective heat transfer between droplets and air is analyzed in a counter-current spray flow heat exchanger. Numerical results for the variations of temperature, velocity and heat transfer rate are presented for two cases of CuCl flow. The optimal dimensions of the heat exchanger are found to be a diameter of 0.13 m, with a height of 0.6 and 0.8 m, for 1 and 0.5 mm droplet diameters, respectively. Additional results are presented and discussed for the heat transfer effectiveness and droplet solidification during heat recovery from the molten CuCl droplets. 相似文献
4.
This paper describes heat and mass transfer characteristics of organic sorbent coated on heat transfer surface of a fin-tube
heat exchanger. The experiments in which the moist air was passed into the heat exchanger coated with sorption material were
conducted under various conditions of air flow rate (0.5–1.0 m/s) and the temperature of brine (14–20°C) that was the heat
transfer fluid to cool the air flow in the dehumidifying process. It is found that the sorption rate of vapor is affected
by the air flow rate and the brine temperature. Meanwhile, the attempt of clarifying the sorption mechanism is also conducted.
Finally the average mass transfer coefficient of the organic sorbent coated on heat transfer surface of a fin-tube heat exchanger
is non-dimensionalzed as a function of Reynolds number and non-dimensional temperature, and it is found that the effect of
non-dimensional temperature on them is larger than Reynolds number . 相似文献
5.
An experimental and numerical investigation of heat transfer and fluid flow was conducted for corrugated-undulated plate
heat exchanger configurations under transitional and weakly turbulent conditions. For a given geometry of the corrugated plates
the geometrical characteristics of the undulated plates, the angle formed by the latter with the main flow direction, and
the Reynolds number were made to vary. Distributions of the local heat transfer coefficient were obtained by using liquid-crystal
thermography, and surface-averaged values were computed; friction coefficients were measured by wall pressure tappings. Overall
heat transfer and pressure drop correlations were derived.
Three-dimensional numerical simulations were conducted by a finite-volume method using a low-Reynolds number k–ɛ model under the assumption of fully developed flow. Computed flow fields provided otherwise inaccessible information on
the flow patterns and the mechanisms of heat transfer enhancement.
Received on 5 February 1999 相似文献
6.
Laminar, fully developed flow and heat transfer between parallel plates with longitudinal fins are analyzed. A modified eigenfunction expansion and point-match method gives highly accurate results. The resistance product f Re and Nusselt numbers for both H1 and H2 problems are determined as a function of fin length and spacing. It is possible to decrease both size and weight of the heat exchanger by the addition of fins. 相似文献
7.
Since the heat transfer performance of syngas cooler affects the efficiency of the power generating system with integrated coal gasification combined cycle (IGCC) directly, it is important to obtain the heat transfer characteristics of high-pressure syngas in the cooler. Heat transfer in convection cooling section of pressurized coal gasifier with the membrane helical coils and membrane serpentine tubes under high pressure is experimentally investigated. High pressure single gas (He or N 2) and their mixture (He + N 2) gas serve as the test media in the test pressure range from 0.5 MPa to 3.0 MPa. The results show that the convection heat transfer coefficient of high pressure gas is influenced by the working pressure, gas composition and symmetry of flow around the coil, of which the working pressure is the most significant factor. The average convection heat transfer coefficients for various gases in heat exchangers are systematically analyzed, and the correlations between Nu and Re for two kinds of membrane heat exchangers are obtained. The heat transfer coefficient of heat exchanger with membrane helical coils is greater than that of the membrane serpentine-tube heat exchanger under the same conditions. The heat transfer coefficient increment of the membrane helical-coil heat exchanger is greater than that of the membrane serpentine-tube heat exchanger with the increase of gas pressure and velocity. 相似文献
8.
Microstructure heat exchangers have unique properties that make them useful for numerous scientific and industrial applications. The power transferred per unit volume is mainly a function of the distance between heat source and heat sink—the smaller this distance, the better the heat transfer. Another parameter governing for the heat transfer is the lateral characteristic dimension of the heat transfer structure; in the case of microchannels, this is the hydraulic diameter. Decreasing this characteristic dimension into the range of several 10s of micrometers leads to very high values for the heat transfer rate. Another possible way of increasing the heat transfer rate of a heat exchanger is changing the flow regime. Microchannel devices usually operate within the laminar flow regime. By changing from microchannels to three dimensional structures, or to planar geometries with microcolumn arrays, a significant increase of the heat transfer rate can be achieved. Microheat exchangers in the form of both microchannel devices (with different hydraulic diameters) and microcolumn array devices (with different microcolumn layouts) are presented and compared. Electrically heated microchannel devices are presented, and industrial applications are briefly described. 相似文献
9.
This review article places in perspective the new work devoted both to the analysis of the thermodynamic irreversibility of heat and mass transfer components and systems and to the design of these devices on the basis of entropy generation minimization. The review focuses on the fundamental mechanisms responsible for the generation of entropy in heat and fluid flow and on the design tradeoff of balancing the heat transfer irreversibility against the fluid flow irreversibility. Applications are selected from the fields of heat exchanger design, thermal energy storage, and mass exchanger design. This article provides a comprehensive, up-to-date review of second-daw analyses published in the heat and mass transfer literature during the last decade. 相似文献
10.
The results of an experimental investigation of the heat transfer coefficients for forced convection from a NACA-63421 airfoil are presented. Wind tunnel measurements of convection coefficients are obtained for air flow temperatures from −30 to 20 °C. The experimental data is correlated with respect to the Nusselt and Reynolds numbers. Conduction within the airfoil balances heat transfer by convection from the airfoil surface in steady-state conditions. Both average and spatial variations of the heat transfer coefficients are non-dimensionalized through modifications of a classical Hilpert correlation for cylinders in crossflow. It is shown that the functional form of the Hilpert correlation can effectively accommodate measured data for the NACA airfoil over a range of Reynolds numbers. An uncertainty analysis is performed to yield a 7.34% measurement uncertainty for experimental data correlated with the Nusselt number. 相似文献
11.
In the present study, the heat transfer characteristics in dry surface conditions of a new type of heat exchanger, namely
a helically coiled finned tube heat exchanger, is experimentally investigated. The test section, which is a helically coiled
fined tube heat exchanger, consists of a shell and a helical coil unit. The helical coil unit consists of four concentric
helically coiled tubes of different diameters. Each tube is constructed by bending straight copper tube into a helical coil.
Aluminium crimped spiral fins with thickness of 0.5 mm and outer diameter of 28.25 mm are placed around the tube. The edge
of fin at the inner diameter is corrugated. Ambient air is used as a working fluid in the shell side while hot water is used
for the tube-side. The test runs are done at air mass flow rates ranging between 0.04 and 0.13 kg/s. The water mass flow rates
are between 0.2 and 0.4 kg/s. The water temperatures are between 40 and 50°C. The effects of the inlet conditions of both
working fluids flowing through the heat exchanger on the heat transfer coefficients are discussed. The air-side heat transfer
coefficient presented in term of the Colburn J factor is proportional to inlet-water temperature and water mass flow rate.
The heat exchanger effectiveness tends to increase with increasing water mass flow rate and also slightly increases with increasing
inlet water temperature. 相似文献
12.
Accurate and reliable dimensionless heat transfer characteristic is very essential for the analysis of heat exchangers. It is also required for the rating and sizing problems of heat exchangers. One of the important experimental methods used to determine the heat transfer coefficient between the heat transfer surface of the heat exchanger and the flowing fluid is transient test techniques. The transient test techniques are usually employed to establish Colburn factor versus Reynolds number characteristics of a high NTU heat exchanger surfaces like compact or matrix heat exchangers. In those situations, a single-blow test, where only one fluid is used, is employed to conduct the transient test. The transient technique may have the fluid inlet temperature having a step change, periodic or an arbitrary rise/drop. In this paper, various transient test techniques that are used for the determination of heat transfer characteristics of high NTU heat exchanger surfaces are discussed. 相似文献
13.
The aim of this paper is to present the formulas for computing the effectiveness and spatial temperature distribution of each stream and the wall of the two-pass countercrossflow heat exchangers with both fluids unmixed throughout for all possible flow arrangements. Making the usual idealizations for analysis of any heat exchanger flow arrangement and giving the coupling conditions for each pass, the problem of finding the spatial temperature distributions in the crossflow heat exchanger core is reduced to the solution of Fredholm's second order integral equation. By using the collocation method the solution of this integral equation is obtainable in the form of power series. The explicit formulas for the spatial temperature distributions and effectiveness are then obtained by simple integrations. The new relations are particularly helpful for computer-aided design procedures of two-pass countercrossflow heat exchangers. 相似文献
14.
The paper presents the results of an investigation concerning heat transfer and pressure loss in air crossflow of diagonally
shaped membrane heating surfaces. The heat and mass transfer analogy by means of naphthalene sublimation technique is used
in order to evaluate mean Nusselt number values in such tube banks. The effect of tube bank arrangement on heat transfer coefficients
and flow resistance is discussed. Convective heat transfer and pressure loss characteristics of diagonally shaped membrane
tube banks, plain tube banks and ordinary membrane tube banks are compared.
Received on 15 June 2000 / Published online: 29 November 2001 相似文献
15.
In the present study, sizing of a single pass cross flow heat exchanger with unmixed fluid streams has been investigated.
The heat exchanger is a cross flow heat exchanger. It has overall dimensions of 20 × 20 × 20 cm. Two the most common heat
exchanger design problems are the rating and sizing problem. Sizing problems deal with designing an exchanger and determining
its physical size to meet the specified heat duty, pressure drops and other considerations. It means the determination of
the exchanger construction type, flow arrangement, heat transfer surface geometries and materials, and the physical sizes
of an exchanger to meet specified heat transfer and pressure drop. In this study, the physical size (length, width, height,
mass flow rates of both fluids and surface areas on each side of the exchanger) are determined. Inputs to the sizing problem
are surface geometries, fluid mass flow rates, inlet and outlet fluid temperatures and pressure drop on each side. Dimensions
of L
a
, L
b
, and L
c
for the selected surfaces were investigated such that the design meets the heat duty and pressure drops on both sides exactly. 相似文献
16.
A numerical investigation of the mixed convection heat transfer from vertical helically coiled tubes in a cylindrical shell at various Reynolds and Rayleigh numbers, various coil‐to‐tube diameter ratios and non‐dimensional coil pitches was carried out. The particular difference in this study compared with other similar studies is the boundary conditions for the helical coil. Most studies focus on constant wall temperature or constant heat flux, whereas in this study it was a fluid‐to‐fluid heat exchanger. The purpose of this article is to assess the influence of the tube diameter, coil pitch and shell‐side mass flow rate on shell‐side heat transfer coefficient of the heat exchanger. Different characteristic lengths were used in the Nusselt number calculations to determine which length best fits the data and finally it has been shown that the normalized length of the shell‐side of the heat exchanger reasonably demonstrates the desired relation. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
17.
In this paper, a numerical investigation is performed for three-stage heat exchangers with plain plate fins and slit fins
respectively, with a three-dimensional laminar conjugated model. The tubes are arranged in a staggered way, and heat conduction
in fins is considered. In order to save the computer resource and speed up the numerical simulation, the numerical modeling
is carried out stage by stage. In order to avoid the large pressure drop penalty in enhancing heat transfer, a slit fin is
presented with the strip arrangement of “front coarse and rear dense” along the flow direction. The numerical simulation shows
that, compared to the plain plate fin heat exchanger, the increase in the heat transfer in the slit fin heat exchanger is
higher than that of the pressure drop, which proves the excellent performance of this slit fin. The fluid flow and heat transfer
performance along the stages is also provided. 相似文献
18.
In the present study, the potential of rectangular fins with 30° and 90° angle and 10 mm offset from the horizontal direction for heat transfer enhancement in a plate fin heat exchanger is numerically evaluated with conjugated heat transfer approach. The rectangular fins are mounted on the flat plate channel. The numerical computations are performed by solving a steady, three-dimensional Navier–Stokes equation and an energy equation by using Fluent software program. Air is taken as working fluid. The study is carried out at Re = 400 and inlet temperatures, velocities of cold and hot air are fixed as 300, 600 K and 1.338, 0.69 m/s, respectively. Colburn factor j versus Re design data is presented by using Fluent. The results show that the heat transfer is increased by 10 % at the exit of channel with fin angle of 30° when compared to channel without fin for counter flow. The heat transfer enhancement with fins of 30° and 90° for different values of Reynolds number with 300, 500 and 800 and for varying fin heights, fin intervals and also temperature distributions of fluids on the top and bottom surface of the channel are investigated for parallel and counter flow. 相似文献
19.
This experimental study comparatively examined the two-phase flow structures, pressured drops and heat transfer performances for the cocurrent air–water slug flows in the vertical tubes with and without the spiky twisted tape insert. The two-phase flow structures in the plain and swirl tubes were imaged using the computerized high frame-rate videography with the Taylor bubble velocity measured. Superficial liquid Reynolds number ( ReL) and air-to-water mass flow ratio (AW), which were respectively in the ranges of 4000–10000 and 0.003–0.02 were selected as the controlling parameters to specify the flow condition and derive the heat transfer correlations. Tube-wise averaged void fraction and Taylor bubble velocity were well correlated by the modified drift flux models for both plain and swirl tubes at the slug flow condition. A set of selected data obtained from the plain and swirl tubes was comparatively examined to highlight the impacts of the spiky twisted tape on the air–water interfacial structure and the pressure drop and heat transfer performances. Empirical heat transfer correlations that permitted the evaluation of individual and interdependent ReL and AW impacts on heat transfer in the developed flow regions of the plain and swirl tubes at the slug flow condition were derived. 相似文献
20.
This paper presents a combined finite element method for solving conjugate heat transfer problems where heat conduction in
a solid is coupled with heat convection in viscous fluid flow. The streamline upwind finite element method is used for the
analysis of thermal viscous flow in the fluid region, whereas the analysis of heat conduction in solid region is performed
by the Galerkin method. The method uses the three-node triangular element with equal-order interpolation functions for all
the variables of the velocity components, the pressure and the temperature. The main advantage of the proposed method is to
consistently couple heat transfer along the fluid-solid interface. Three test cases, i.e. conjugate Couette flow problem in
parallel plate channel, counter-flow in heat exchanger, and conjugate natural convection in a square cavity with a conducting
wall, are selected to evaluate the efficiency of the present method.
The English text was polished byYunming Chen. 相似文献
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