Determination of heat transfer correlations for plate-fin-and-tube heat exchangers

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.     

Transient heat transfert in coflow heat exchanger

More than ever before, dynamic investigation techniques are becoming widely used in the control systems, parameter implementation and state estimators. Indeed, dynamical models describing the response of process systems that are subject to disturbances play a vital role in controlling and optimising these systems. Recently developed in literature, the method of step response analysis provides a promising means towards solving some of the problems associated with the characterisation of transient response of heat exchangers. In Abdelghani-Idrissi et al. (Int J Heat Mass Transfer 44:3721–3730, 2001), authors present analytical expressions of fluids temperatures response time of counter-current heat exchanger when hot fluid step change is applied in the internal tube. This paper describes the extension of this technique to a coflow heat exchanger for which the exact solution of its mathematical model is unavailable.     

Radiant heat gain from indoor heat sources

A new parameter, equivalent emissivity, ε_e, is introduced to reflect radiant feature of the surface of a heat source with different shapes and different temperatures in an air conditioned room. Radiant heat gain from medium- or low-surface-temperature equipment and from commonly used light fixtures and the occupant are examined and analyzed in detail. The main affecting factors are also examined. In addition, the percentages of radiant heat gain are presented under different temperatures.     

Numerical simulation of turbulent heat transfer from perforated plate-fin heat sinks

Excessive heat from microelectronic components is essential to remove to increase the reliability of the system. In this paper, various types of perforations in the form of small channels such as square, circular, triangular and hexagonal cross sections are introduced and thermal performances are compared to improve the cooling performance of heat sink. The governing equations are solved by adopting a control volume based finite element method with an unstructured non-uniform grid system. Flow and heat transfer characteristics are presented for Reynolds numbers from 2 × 10⁴ to 4 × 10⁴ based on the fin length and Prandtl number is taken as Pr = 0.71. RANS based k-ε turbulence model is used to predict the turbulent flow parameters. The predicted results are validated by the previously published experimental data and in reasonable agreement with the experiment. Results show that fins having circular perforations have better thermal and fluid dynamic performances than the other types of fins considered here.     

Three-dimensional numerical analysis of heat and mass transfer in heat pipes

A three-dimensional finite-element numerical model is presented for simulation of the steady-state performance characteristics of heat pipes. The mass, momentum and energy conservation equations are solved for the liquid and vapor flow in the entire heat pipe domain. The calculated outer wall temperature profiles are in good agreement with the experimental data. The estimations of the liquid and vapor pressure distributions and velocity profiles are also presented and discussed. It is shown that the vapor flow field remains nearly symmetrical about the heat pipe centerline, even under a non-uniform heat load. The analytical method used to predict the heat pipe capillary limit is found to be conservative.     

Concepts and realization of microstructure heat exchangers for enhanced heat transfer

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.     

Three-dimensional numerical investigation of heat transfer for plate fin heat exchangers

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.     

Investigation of flow and heat transfer in corrugated-undulated plate heat exchangers

 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     

Two phase flow and heat transfer characteristics of a separate-type heat pipe

Two phase flow and heat transfer characteristics of a separate-type heat pipe have been studied experimentally and theoretically. The experimental apparatus have the same geometry for the evaporator and the condenser which consist of 5-tube-banks, with working temperature ranges of 80–125°C. The experimental working fluid is dual-distilled water with corrosion-resistant agents. Heat transfer coefficients for boiling and condensation along with heat flux and working temperature are measured at different filling ratio. According to the results of the experiments, the optimized filling ratio ranges from 16 to 36%. Fitted correlations of average heat transfer coefficients of the evaporator and Nusselt numbers of the condenser at the proposed filling ratio are obtained. Two phase flow characteristics of the evaporator and the condenser as well as their influence on heat transfer are described on the basis of simplified analysis. Reasons for the pulse-boiling process remain to be studied.     

Effects of thermal dispersion on heat transfer in cross-flow tubular heat exchangers

Effects of thermal dispersion on heat transfer and temperature field within cross-flow tubular heat exchangers are investigated both analytically and numerically, exploiting the volume averaging theory in porous media. Thermal dispersion caused by fluid mixing due to the presence of the obstacles plays an important role in enhancing heat transfer. Therefore, it must be taken into account for accurate estimations of the exit temperature and total heat transfer rate. It is shown that the thermal dispersion coefficient is inversely proportional to the interstitial heat transfer coefficient. The present analysis reveals that conventional estimations without consideration of the thermal dispersion result in errors in the fluid temperature development and underestimation of the total heat transfer rate.     

Investigation of heat transfer on the Dowthern A gravity assisted heat pipe

An experimental study on heat transfer performance of a gravity-assisted heat pipe is described. Dowtherm A has been used as working fluid. The effect of fill ratio, inclination angle, heat length ratio, and thermal resistance on the rate of heat flow were investigated. Finding a dependence of the maximum rate of heat flow on the first two parameters, a corresponding numerical correlation has been formulated. The force of viscosity shear and the force of momentum transfer during the condensation have been accounted for, and, with a theoretical analysis and a numerical solution, a mean heat transfer coefficient in the condensation section is given.

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Untersuchung der Wärmeübertragung in einem Gravitationswärmerohr mit Dowtherm A als Arbeitsfluid

Zusammenfassung Es wird eine experimentelle Untersuchung des Wärmeübertragungsvermögens eines Gravitationswärmerohres beschrieben. Als Arbeitsfluid diente Dowtherm A. Der Einfluß der eingefüllten Menge, des Neigungswinkels, der beheizten Länge und des Wärmewiderstandes auf den Wärmestrom wurde untersucht. Hierbei zeigte sich eine Abhängigkeit des maximalen Wärmestroms von den ersten beiden Parametern, die sich als Gleichung formulieren ließ. Unter Berücksichtigung der viskosen Scherkräfte und der Kräfte infolge Impulsübertragung während der Kondensation wird neben einer theoretischen Analyse und der numerischen Lösung der mittlere Wärmeübergangskoeffizient im Kondensationsgebiet angegeben.

     

Calculation of overall heat transfer coefficients in a triple tube heat exchanger

In previous studies, calculation of overall heat transfer coefficients in a triple tube heat exchanger (TTHE) involved assumptions or approaches those are not valid in all cases. In this study a more generic way of calculating overall heat transfer coefficients in a TTHE has been developed. Consequently, temperature profiles of all streams in a TTHE in the axial direction were determined. An effective overall heat transfer coefficient that is related to the total resistance to heat transfer in the TTHE, was also determined to facilitate comparison of a TTHE to an equivalent double tube heat exchanger.     

Topology optimization and heat dissipation performance analysis of a micro-channel heat sink

Junction temperature in the electronic packaging process is one of the critical factors affecting the service life of electronic devices. A micro-channel heat sink is a common heat dissipating device used to reduce the thermal resistance between components and substrate. In order to maximize the heat dissipation while minimizing the pressure drop, this paper adopts a topology optimization method. A material interpolation method based on variable density principle is used together with a moving asymptote algorithm for the optimization. The physics is governed by the heat and mass transfer, coupled with the momentum conservation in the fluid. Four parameters are varied in order to investigate their influence on the optimization process. A three-dimensional geometry has been constructed to study the flow field and the results are compared to a reference case to verify the temperature uniformity and thermal performance of the model. It is demonstrated that the optimized design of the micro-channel heat sink is reliable and effective.