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1.
The present experimental and numerical work investigates the effect of metallic porous materials, inserted in a pipe, on the rate of heat transfer. The pipe is subjected to a constant and uniform heat flux. The effects of porosity, porous material diameter and thermal conductivity as well as Reynolds number on the heat transfer rate and pressure drop are investigated. The results are compared with the clear flow case where no porous material was used. The results obtained lead to the conclusion that higher heat transfer rates can be achieved using porous inserts at the expense of a reasonable pressure drop. Also, it is shown that for an accurate simulation of heat transfer when a porous insert is employed its effective thermal conductivity should be carefully evaluated.  相似文献   

2.
A three-dimensional inverse problem in determining the local heat transfer coefficients for the plate finned-tube heat exchangers utilizing the steepest descent method (SDM) and a general purpose commercial code CFX4.4 is applied successfully in the present study based on the measured temperature distributions on fin surface by infrared thermography.Two different tube arrangements (i.e. in-line and staggered) with different fin pitch and air velocity are considered and the corresponding local heat transfer coefficients are to be determined. Results show that some interesting phenomena of the local heat transfer coefficients for the finned surface are found in the work and the averaged heat transfer coefficient of the staggered configuration is about 8–13% higher than that of the in-line configuration.  相似文献   

3.
Using the conservation equations for mass, momentum and energy, a theoretical analysis of buoyancy driven flow and heat transfer for a ventilated control-volume, with an internal heat-source, has been made. The special case of stoichiometric combustion in a naturally-ventilated brick walled room, with a single rectangular opening, has been used to demonstrate the numerical calculation procedure for the prediction of the histories of the fire temperature, gas flow rate, fuel burn rate, fire power and boundary-wall temperature. The analysis may be extended for more complex space geometries and wall structures; a typical case being a railway carriage with a composite wall.  相似文献   

4.
In this paper, heat transfer of single cylindrical particle affected by wall has been investigated numerically and experimentally for Reynolds number range 2000 to 6000. The heat transfer in two different orientations, axial and cross flow over the particle has been considered in simulation with MultiPhysics Software FEMLAB version 2.3. The heat and mass transfer analogy technique has been applied for validation of the simulation results. The coated particle with naphthalene was sublimated to obtain the corresponding Sherwood numbers. The results show that the CFD model can predict the particle-to-fluid heat transfer for two situations due to trivial error (an average error of 6%) compared to experimental values. Influence of wall on heat transfer of particle in seven different bed-to-cylinder diameter ratio (N = 1.66, 2.65, 2.75, 5, 6.66, 12, and 18) have been discussed in different velocities. According to obtaining results, with increasing the bed-to-cylinder diameter ratio over the 12 wall have no significant consequence on Nusselt number. Due to this fact, a CFD based correlation has been proposed to consider the wall effects on particle-to-fluid Nusselt number with an average error of 2.19%.  相似文献   

5.
A pulsating flow in a pipe was experimentally investigated to determine the effect of pulsation on the rate of heat transfer. The influence of hydrodynamic parameters and characteristics of the pulsation on heat transfer was carefully studied. In order to adjust the pulsating parameters, a self‐oscillator was designed so the length of the resonator and the length of the outlet nozzle could be adjusted. The results show that the heat transfer rate is strongly affected by both the hydrodynamic parameters and the configuration of the resonator. With the increase of the flow rate of the liquid and the length of the chamber, heat transfer is enhanced. There is an optimal length at which the heat transfer enhancement attends to the best. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(5): 279–286, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20020  相似文献   

6.
An experimental study has been conducted on the heat transfer of oscillating flow through a channel filled with aluminum foam subjected to a constant wall heat flux. The surface temperature distribution on the wall, velocity of flow through porous channel and pressure drop across the test section were measured. The characteristics of pressure drop, the effects of the dimensionless amplitude of displacement and dimensionless frequency of oscillating flow on heat transfer in porous channel were analyzed. The results revealed that the heat transfer in oscillating flow is significantly enhanced by employing porous media in a plate channel. The cycle-averaged local Nusselt number increases with both the kinetic Reynolds number Reω and the dimensionless amplitude of flow displacement A0. The length-averaged Nusselt number is effectively increased by increasing the kinetic Reynolds number from 178 to 874 for A0 = 3.1-4.1. Based on the experimental data, a correlation equation of the length-averaged Nusselt number with the dimensionless parameters of Reω and A0 is obtained for a porous channel with L/Dh = 3.  相似文献   

7.
An experimental study was performed on heat transfer of an open thermosyphon with constant wall heat flux. Water and aqueous glycerin were used as working fluids. The experimental range of modified Rayleigh number was 1 × 103 < Ram < 3 × 105. The average and local heat transfer coefficients, vertical temperature distributions of the tube wall and fluid at the centerline of the tube, and temperature fluctuations of the fluid were measured. Flow patterns were observed by adding tracer powder to the fluid. Fluid velocities were measured by laser Doppler velocimeter. Experimental results indicate that, for a water thermosyphon, there are three regimes where different heat transfer characteristics and flow patterns occur. For 1 × 103 < Ram < 3 × 103, the flow was laminar and the thermal boundary layer reached the center of the tube. Heat was exchanged between the wall and descending flow. Wall temperature increased in the downward direction. For 4 × 103 < Ram < 3 × 104, no turbulence was observed in the flow and the thermal boundary layer was localized in the vicinity of the wall. The wall temperature increased upward. For 3 × 104 < Ram < 3 × 105, flow was considerably disturbed by the mixing of upward and downward flow in the upper part of the tube. However, the flow was laminar in the lower part of the tube. Reduction of the flow rate induced by the flow mixing at high Ram can be one of the major causes of the deterioration of heat transfer from Lighthill's theory. © 2001 Scripta Technica, Heat Trans Asian Res, 30(4): 301–312, 2001  相似文献   

8.
The effect of SiO2 particles on heat transfer performance of a pulsating heat pipe(PHP) was investigated experimentally.DI water was used as the base fluid and contrast medium for the PHP.In order to study and measure the character,there are SiO2 /H2 O nanofluids with different concentration and applying with various heating powers during the experiment investigation.According to the experimental result,the high fraction of SiO2 /H2 O will deteriorate the performance of PHP compared with DI water,i.e.the thermal resistance and the temperature of evaporation section increases.It is in contrary in the case of low fraction of SiO2 /H2 O.Finally,the comparison of the thermal performances between the normal operation system and the static settlement system is given.It is found that both the thermal resistance of nanofluid PHP and the temperature of the evaporation section increase after standing for a period,and it is the same trend for the temperature fluctuation at the identical heating power for PHP.  相似文献   

9.
Thermal treatment of fluid foods represents a major unit operation in the food industry, to ensure the product's safety and quality features. But during the thermal treatments of such sensible fluids in common plate heat exchangers, food constituents such as proteins can be thermally damaged and precipitated to form fouling that greatly affect the treatment efficiency and alter the product's desired features.Computational Fluid Dynamics simulations can then be successfully exploited, bringing forth temperature and velocity information that yield for deposit distributions when coupled to biochemical notations for thermal denaturation of fluid constituents.The present work exploits such modeling for a single-channel heat exchanger during pasteurization of milk. The model enforces a conjugate system of differential equations to a heat exchanger's corrugated plate to combine flow, heat transfer and local transport of β-lactoglobulin. A preliminary computation has been performed that could be applied to geometry optimization (different corrugation shape and orientation) and for a variety of biochemically evolutive products.  相似文献   

10.
An experimental investigation was conducted to study the effect of pin fins on heat transfer in circulating fluidized beds. Experiments were conducted in a 100 mm i.d., 5.15 m tall CFB unit, initially with no fins and then with 16-pin and 32-pin fins. Each pin was 6.35 mm in diameter and 15 mm long. It was observed that, although the heat transfer coefficient decreases with the use of fins, the total heat transfer increases owing to the increase in surface area which the fins provide. The results of heat transfer for unfinned surface were compared with those of other workers and found to be in good agreement.  相似文献   

11.
In the present study an experimental investigation of the mixed convection heat transfer in a coil-in-shell heat exchanger is reported for various Reynolds and Rayleigh numbers, various tube-to-coil diameter ratios and dimensionless coil pitch. The purpose of this article is to assess the influence of the tube diameter, coil pitch, shell-side and tube-side mass flow rate over the performance coefficient and modified effectiveness of vertical helical coiled tube heat exchangers. The calculations have been performed for the steady-state and the experiments were conducted for both laminar and turbulent flow inside coil. It was found that the mass flow rate of tube-side to shell-side ratio was effective on the axial temperature profiles of heat exchanger. The results also indicate that the ? − NTU relation of the mixed convection heat exchangers was the same as that of a pure counter-flow heat exchanger.  相似文献   

12.
以烃类物质(丙烷和正戊烷)作为工质,进行了紧凑式换热器中带有加工配置表面的管式换热元件池沸腾实验研究。其中,单管实验温度工况为253K ̄293K(饱和工质)。实验中所采用的换热元件为重入式结构加工配置表面的强化传热管和光管以及低助管。针对由45根光管或带有加工配置表面的管子所构成的叉排管束进行了实验研究,实验工质为丙烷和正戊烷,实验温度分别为两种工质在263K和308K之间的饱和和温度。并将所得实  相似文献   

13.
Numerical computation of fluid flow and heat transfer in microchannels   总被引:12,自引:0,他引:12  
Three-dimensional fluid flow and heat transfer phenomena inside heated microchannels is investigated. The steady, laminar flow and heat transfer equations are solved using a finite-volume method. The numerical procedure is validated by comparing the predicted local thermal resistances with available experimental data. The friction factor is also predicted in this study. It was found that the heat input lowers the frictional losses, particularly at lower Reynolds numbers. At lower Reynolds numbers the temperature of the water increases, leading to a decrease in the viscosity and hence smaller frictional losses.  相似文献   

14.
A solar chimney power plant consists of a translucent collector which heats the air near the ground and guides it into the base of a chimney at its centre. The buoyant air rises in the chimney and electricity is generated through one or more turbines in or near the base of the chimney. Various studies about solar chimney power plant performance have been published. Different calculation approaches with a variety of considerations have been applied to calculate chimney power plant performance. In particular, two comprehensive studies are relevant, namely those of (Bernardes, M.A.d. S., Voß, A., Weinrebe, G., 2003. Thermal and technical analyses of solar chimneys. Solar Energy 75, 511-524; Pretorius, J.P., Kröger, D.G., 2006b. Solar chimney power plant performance. Transactions of the ASME 128, 302-311). The paper compares the methods used to calculate the heat fluxes in the collector, and their effects on solar chimney performance. Reasons for the discrepancies between the predictions of the two models are given. In general the Pretorius model produces higher heat transfer coefficients and higher heat rate fluxes for both the roof and for the ground surfaces. The two approaches lead to very similar air temperature rises in the collector and thus, similar produced power.  相似文献   

15.
Experimental results of convection heat transfer to supercritical carbon dioxide in heated horizontal and vertical miniature tubes are reported in this paper. Stainless steel circular tubes having diameters of 0.70, 1.40, and 2.16 mm were investigated for pressures ranging from 74 to 120 bar, temperatures from 20 to 110 °C, and mass flow rates from 0.02 to 0.2 kg/min. The corresponding Reynolds numbers and Prandtl numbers ranged from 104 to 2×105 and from 0.9 to 10, respectively. It is found that the buoyancy effects were significant for all the flow orientations, although Reynolds numbers were as high as 105. The experimental results reveal that in downward flow, a significant impairment of heat transfer was discerned in the pseudocritical region, although heat transfer for both horizontal and upward flow was enhanced. The experimental results further indicate that in all the flow orientations, the Nusselt numbers decreased substantially as the tube diameter shrunk to <1.0 mm. Based on the experimental data, correlations were developed for the axially-averaged Nusselt number of convection heat transfer to supercritical carbon dioxide in both horizontal and vertical miniature heated tubes.  相似文献   

16.
A numerical investigation was conducted to analyze the unsteady turbulent flowfield and heat transfer characteristics in a channel with streamwise periodically mounted square bars arranged side-by-side to the approaching flow. The transverse separation distance between the bars is varied, whereas the bar height to channel height (d/H) are 0.152 and 0.2, the Reynolds number Re based on channel height is 2×104 and the periodicity length is 2H. The channel walls are subjected to a constant wall temperature. The k-ε turbulence model was used in conjunction with the Reynolds-averaged momentum and energy equations for the simulations. A finite volume technique is applied with a fine grid and time resolution. Complex periodic vortex shedding develops in the channel due the interaction between the two streamwise periodically mounted square bars. Results show that the unsteady flow behavior, pressure drop and heat transfer are strongly dependent of the transverse separation distance of the bars.  相似文献   

17.
This paper reports the experimental and Computational Fluid Dynamics (CFD) modeling studies on heat transfer, friction factor and thermal performance of an air cooled heat exchanger equipped with three types of tube insert including butterfly, classic and jagged twisted tape. In the studied range of Reynolds number the maximum thermal performance factor was obtained by the butterfly insert with an inclined angle of 90°. The results have also revealed that the difference between the heat transfer rates obtained from employing the classic and jagged inserts reduces by decreasing the twist ratio. The CFD predicted results were used to explain the observed results in terms of turbulence intensity. In addition, good agreements between the predicted and measured Nu number as well as friction factor values were obtained.  相似文献   

18.
In this study, experimental and three dimensional numerical work was carried out to determine the average heat transfer coefficients for forced convection air flow over a rectangular flat plate. Three dimensional numerical simulations were obtained using a commercial finite volume based fluid dynamics code called Fluent 6.3. The experiments were performed for mass transfer using the naphthalene sublimation technique. The results were presented in terms of heat transfer parameters using the analogy between heat and mass transfer. All the experimental results are correlated within an accuracy of ± 12%.  相似文献   

19.
A numerical work has been conducted to examine turbulent periodic flow and heat transfer characteristics in a three dimensional square-duct with inline 60° V-shaped discrete thin ribs placed on two opposite heated walls. The isothermal-flux condition is applied only to the upper and lower duct walls while the two sidewalls are insulated, similar to internal passage cooling of gas turbine blades. The computations are based on the finite volume method with the SIMPLE algorithm for handling the pressure–velocity coupling. Air is the working fluid with the flow rate in terms of Reynolds numbers ranging from 10,000 to 25,000. The numerical result is validated with available square-rib measured data and found to agree well with measurement. The computation reveals that the ribbed duct flow is fully developed periodic flow and heat transfer profiles at about x/D = 7–11 downstream of the inlet. Effects of different rib height to duct diameter ratios, BR, on thermal characteristics for a periodic ribbed duct flow are investigated. It is found that a pair of counter-rotating vortices (P-vortex) caused by the rib can induce impingement/attachment flows on the walls leading to greater increase in heat transfer over the test duct. In addition, the rise of BR values leads to the increase in heat transfer and friction loss. The maximum thermal performance is around 1.8 for the rib with BR = 0.0725 where the heat transfer rate is about 4.0 times above the smooth duct at lower Reynolds number.  相似文献   

20.
The problem of simultaneous heat and mass transfer in two-dimensional free convection from a semi-infinite vertical flat plate is investigated. An integral method is used to find a solution for zero wall velocity and for a mass transfer velocity at the wall with small-amplitude oscillatory wall temperature. Low- and high-frequency solutions are developed separately and are discussed graphically with the effects of the parameters Gr (the Grashof number for heat transfer), Gc (the Grashof number for mass transfer) and Sc (the Schmidt number) for Pr = 0–71 representing aid at 20°C.  相似文献   

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