Showing papers on "Micro heat exchanger published in 2001"

A Comparative Analysis of Studies on Heat Transfer and Fluid Flow in Microchannels

Abstract: The extremely high rates of heat transfer obtained by employing microchannels makes them an attractive alternative to conventional methods of heat dissipation, especially in applications related to the cooling of microelectronics. A compilation and analysis of the results from investigations on fluid flow and heat transfer in micro- and mini-channels and microtubes in the literature is presented in this review, with a special emphasis on quantitative experimental results and theoretical predictions. Anomalies and deviations from the behavior expected for conventional channels, both in terms of the frictional and heat transfer characteristics, are discussed.

Nonuniform temperature distribution in electronic devices cooled by flow in parallel microchannels

Abstract: We fabricated a novel thermal microsystem (simulating a computer chip) consisting of a heater, microchannels, inlet and outlet plena and we studied the effect of the geometry on the flow and heat transfer. The vapor-water two-phase flow patterns were observed in the parallel microchannels through a microscope and high-speed video camera. It was observed that hydraulic instabilities occur. Existence of a periodic annular flow was also observed, which consists of a symmetrically distributed liquid ring surrounding the vapor core. Along the microchannel axis, the periodic dry zone appears and develops. The thermal visualization and temperature measurements of the heated device were carried out using infrared thermography. As long as the flow was single phase liquid, the forced convection heat transfer resulted in a moderate irregularity on the heated chip. These temperature differences do not cause damage to a real electronic device. The steady-state heat transfer for different types of microchannels has been studied also at the range of heat flux where phase change of the working fluid from liquid to vapor took place. Under conditions of flow boiling in microchannels, a significant enhancement of heat transfer was established. In the case of uniform heat flux the hydraulic instabilities lead to irregularity of temperature distribution on the heated chip. In the case of nonuniform heat flux the irregularity increased drastically.

Crossflow micro heat exchanger

Abstract: An extremely high efficiency, cross flow, fluid-fluid, micro heat exchanger and novel method of fabrication using electrode-less deposition is disclosed. To concurrently achieve the goals of high mass flow rate, low pressure drop, and high heat transfer rates, the heat exchanger comprises numerous parallel, but relatively short microchannels. Typical channel heights are from a few hundred micrometers to about 2000 micrometers, and typical channel widths are from around 50 micrometers to a few hundred micrometers. The micro heat exchangers offer substantial advantages over conventional, larger heat exchangers in performance, weight, size, and cost. The heat exchangers are especially useful for enhancing gas-side heat exchange. The use of microchannels in a cross-flow micro-heat exchanger decreases the thermal diffusion lengths substantially, allowing substantially greater heat transfer per unit volume or per unit mass than has been achieved with prior heat exchangers.

Two-phase heat transfer to a refrigerant in a 1 mm diameter tube

Abstract: A study of two-phase flow and heat transfer in a small tube of 1 mm internal diameter has been conducted experimentally as part of a wider study of boiling in small channels. R141b has been used as the working fluid. The boiling heat transfer in the small tube has been measured over a mass flux range of 300–2000 kg/m2 s and heat flux range of 10–1150 kW/m2. In this paper the boiling map for a mass velocity of 510 kg/m2 s and heat flux of 18–72 kW/m2 is discussed and the problems of determining heat transfer coefficients in small channels are highlighted.

Microchanneled active fluid heat exchanger method

Abstract: A heat exchanger utilizing active fluid transport of a heat transfer fluid is manufactured with multiple discrete flow passages provided by a simple but versatile construction. The microstructured channels are replicated onto a film layer which is utilized in the fluid transfer heat exchanger. The surface structure defines the flow channels which are generally uninterrupted and highly ordered. These flow channels can take the form of linear, branching or dendritic type structures. A cover layer having favorably thermal conductive properties is provided on the structured bearing film surface. Such structured bearing film surfaces and the cover layer are thus used to define microstructure flow passages. The use of a film layer having a microstructured surface facilitates the ability to highly distribute a potential across the assembly of passages to promote active transport of a heat transfer fluid. The thermally conductive cover layer then effects heat transfer to an object, gas, or liquid in proximity with the heat exchanger.

Modeling of two-phase microchannel heat sinks for VLSI chips

Abstract: Microchannel heat sinks with forced convective boiling can satisfy the increasing heat removal requirements of VLSI chips. But little is known about two-phase boiling flow in channels with cross-sectional dimensions below 100 /spl mu/m. This work develops and experimentally verifies microchannel simulations, which relate the temperature field to the applied power and flowrate. The simulations consider silicon conduction and assume an immediate transition to homogeneous misty flow, without the bubbly and plug-flow regimes in larger channels. Pressure drop and wall temperature predictions are consistent with data for a channel with cross-sectional dimensions of 50 /spl mu/m/spl times/70 /spl mu/m. The simulations explore the performance of a novel heat sink system with an electrokinetic pump for the liquid phase, which provides 1 atm and 15 ml/min. A temperature rise below 40 K is predicted for a 200 W heat sink for a 25 mm/spl times/25 mm chip.

Numerical study of thermoacoustic heat exchangers in the thin plate limit

Abstract: The velocity and temperature fields in an idealized thermoacoustic refrigerator are analyzed computationally. The numerical model simulates the unsteady mass, momentum, and energy equations in the thin-plate, low-Mach-number limits. Two-dimensional unsteady calculations of the flow field in the neighborhood of the stack and heat exchangers are performed using a vorticity-based scheme for stratified flow. The computations are applied to analyze the effects of heat-exchanger length and position on the performance of the device. The results indicate that the cooling load peaks at a well-defined heat-exchanger length, stack gap, and distance between the heat exchangers and the stack plates.

Progress in the numerical analysis of compact heat exchanger surfaces

Abstract: Publisher Summary Compact heat exchangers (CHEs) are characterized by a large heat transfer surface area per unit volume of the exchanger, resulting in reduced space, weight, support structure and footprint, energy requirements, and cost, and improved process design, plant layout, and processing conditions, together with low fluid inventory compared to conventional designs such as shell-and-tube heat exchangers. Plate-fin, tube-fin, and rotary regenerators are examples of compact heat exchangers for gas flow on one or both sides; whereas gasketed, welded, brazed plate, and printed circuit heat exchangers are examples of compact heat exchangers for liquid flows. A considerable amount of experimental results are available in the literature for flow and heat transfer phenomena in complex flow passages of compact heat exchanger surfaces. A comprehensive experimental study of the performance of CHE surfaces is very expensive because of the high cost of the tools needed to produce a wide range of geometric variations. Numerical modeling, on the other hand, has the potential of offering a flexible and cost-effective means for such a parametric investigation, with the added advantage of reproducing ideal geometries and boundary conditions, and exploring the performance behavior in specific and critical areas of flow geometry. This chapter provides a comprehensive state-of-the-art review on numerical studies of single-phase velocity and temperature fields, and heat transfer and flow friction characteristics of compact heat exchanger surfaces, and also provides specific comparisons to evaluate the accuracy of numerical work where experimental data are available.

Development of Micro Channel Heat Exchanging

Abstract: In order to investigate the performance of the micro channel heat exchanger, three-dimensional numerical simulations and experiments on heat transfer behavior and pressure loss were carried out. So far as the heat transfer phenomena is concerned, results obtained using a silicon chip micro channel model showed a very small thermal resistance, about 0.1 (Kcm2/W). And, measured pressure loss showed good agreement with that of analytical result obtained on the basis of fully developed laminar pipe flow assumption. Furthermore, a practical setup was made with a micro channel heat exchanger to clarify the possibility of using the micro channel heat exchanger in electrical equipment. As a result, it was confirmed that the performance of the micro channel heat exchanger system is sufficient to cool a silicon chip which generates a large amount of heat, and the scale of the system is compact compared to that of the whole setup of electrical equipment.

An experimental study on vapor condensation of wet flue gas in a plastic heat exchanger

Abstract: An experimental system investigating condensation heat transfer of wet flue gas was set up, and the heat transfer performance of vapor-gas mixture with vapor condensation was discussed. The experimental results of laminar flow in a plastic longitudinal spiral plate heat exchanger were obtained and are in good agreement with the modified classical film model. It is shown that the plastic air preheater can avoid acid corrosion in the low-temperature field for the boiler using fuel containing sulfur and recover latent heat of the water vapor of the wet flue gas. Also some SO2 was scrubbed during the vapor condensing process in the heat exchanger. © 2001 Scripta Technica, Heat Trans Asian Res, 30(7): 571–580, 2001

Convective heat and mass transfer in tube-fin exchangers under dehumidifying conditions

Abstract: The article analyzes convective heat and mass transfer in the flow passages of tube-fin exchangers, adopting a simplified two-dimensional approach. The flow structure on the airside of these devices is spatially periodic, with fully developed conditions prevailing a short distance from the entrance. In numerical simulations, symmetric and/or antisymmetric periodicity in pressure, velocity components, temperature, and mass concentration of the water vapour are taken into account to reduce the computational domain. Using a finite-element discretization velocity, temperature and mass concentration fields are computed within wavy, offset-strip, and louver fin surfaces. Quantitative results are also obtained for friction factors, Nusselt numbers, and Colburn factors for heat and mass transfer.

Performance of heat-transfer irreversible regenerated Brayton refrigerators

Abstract: The performance of an externally and internally irreversible regenerated Brayton refrigerator is analysed. Analytical relationships between cooling load and pressure ratio, as well as between coefficient of performance (COP) and pressure ratio of a real closed irreversible regenerated Brayton refrigeration cycle coupled to constant- or variable-temperature heat reservoirs are derived. The irreversibilities considered in the analysis include the heat transfer losses in the hot- and cold-side heat exchangers and the regenerator, the non-isentropic expansion and compression losses in the compressor and expander, and the pressure drop losses in the piping system. The optimal performance characteristics of the cycle may be obtained by optimizing the distribution of the heat conductance or heat transfer surface areas among the two heat exchangers and the regenerator. The influences of the effectiveness of the regenerator as well as the hot- and cold-side heat exchangers, the efficiencies of the expander and the compressor, the pressure recovery coefficient, and the inlet temperature ratio of the heat reservoirs on the cooling load and the COP are examined and shown by numerical examples.} \setlength\abswidth{32pc

Numerical predictions of wavy fin coil performance

Abstract: The wavy fin-and-tube heat exchanger is widely used in the air-conditioning industry. Hence, the ability of numerical codes to predict the thermo-hydraulic performance of this heat exchanger geometry is of considerable interest. Here, a numerical study has been conducted to predict the air-side heat transfer and pressure drop characteristics of a 2-row finned tube heat exchanger having a herringbone wavy fin geometry. The calculations were performed using the FLUENT CFD code. The calculations were performed in three dimensions and account for conduction within the fins. The computational domain was chosen to cover the entire fin length in the air flow direction. The calculations are implemented for the frontal air velocities between 1.0 and 3.0 m/s yielding hydraulic diameter Reynolds number from 380 to 1,150. The uniqueness of this work is inclusion of the tubes in the wavy channel geometry, and use of a three-dimensional (3D) calculation model. The flow and thermal fields in this finned tube heat exchanger as well as the temperature distribution of the fin are presented and described. The fin efficiency is numerically determined and compared with that given by the Schmidt equation (1966) and found to agree within {+-}2.3%. Predictions were performed for a herringbone channelmore » without and with tubes. The tubes considerably increase the pressure drop, but the effect on the average heat transfer coefficient is much smaller. The calculation results are compared with test data taken by Kang and Webb (1997) for a test sample having the same geometry. The FLUENT over predicts the air-side heat transfer coefficient by 13% and pressure drop by 31%.« less

Comparison of Crystallization Fouling in Plate and Double-Pipe Heat Exchangers

Abstract: The selection of fouling resistances to design plate-and-frame heat exchangers is usually based on experience or guesswork. The primary aim of this study was to compare fouling resistances for plate heat exchangers with values obtained in a parallel double-pipe heat exchanger under comparable operating conditions. The double-pipe heat exchanger was selected because it represents a shell-and-tube heat exchanger in a simplified form. Correction factors for plate heat exchangers have been obtained by comparing the performance of the two heat exchanger types during calcium sulfate crystallization fouling. Fouling was investigated in both heat exchangers for two conditions: (1) particles present in the process solution were allowed to enhance the crystallization process and (2) particles present in the process solution were removed by an in-line 1-μm filter. For similar flow velocities, the plate heat exchanger was found to foul significantly less than the double-pipe heat exchanger, i.e. 20-25 times less in the presence of particles and around 15 times less when the particles were filtered out. When data for similar overall shear forces are compared, results for both heat exchanger types are much closer, even though the plate heat exchanger still seems to perform somewhat better. Fouling in plate-and-frame heat exchangers was also found to depend significantly on plate design. This will add a further complication in direct comparison with shell-and-tube heat exchangers.

Corrugated heat exchanger element having grooved inner and outer surfaces

Abstract: A corrugated heat exchanger element may have grooves (36) and ribs (38) on the interior and exterior surfaces of the tube. The corrugations on the tube may be linear corrugations or helical corrugations. The texturing of the interior and exterior surfaces (32,34) of the tube, and the corrugations in the tube may provide a heat exchanger element that has a large surface area. The surface texturing and corrugations may also provide a heat exchanger element that promotes internal mixing of fluids that flow by and through the element. Increased internal mixing and increased surface area of an element may allow the heat exchanger element to have a high heat transfer coefficient. A heat exchanger element (30) or heat exchanger elements may be assembled into a heat exchanger that has a high overall heat transfer coefficient. Ends of the heat exchanger element may be pointed to facilitate attachment of the element to a support structure.

Air conditioner with internal heat exchanger and heat exchanger tube therefor

Abstract: An air-conditioning system for a motor vehicle contains a heat exchanger with a heat-exchanger tube that features a profiled central channel and outer channels grouped around the central channel. This heat-exchanger tube is suitable for the construction of counterflow heat exchangers. For this purpose, the heat-exchanger tube is to be cut to corresponding lengths and provided with corresponding end pieces. Such heat exchangers have proven to be light, pressure resistant, and effective.

Design of shell-and-tube heat exchangers by using CFD technique, Part one: Thermo-hydraulic calculation

Abstract: In this paper, an iterative procedure for sizing shell-and-tube heat exchangers according to prescribed pressure drop is shown, then the thermo-hydraulic calculation and the geometric optimization for shell and tube heat exchangers on the basis of CFD technique have been carried out. Modeling of shell and tube heat exchangers for design and performance evaluation is now an established technique used in industry. In this paper, a numerical study of three-dimensional fluid flow and heat transfer in a shell and tube model heat exchanger is described. The baffle and tube bundle was modeled by the 'porous media' concept. Three turbulent models were used for the flow processes. The velocity and temperature distributions as well as the total heat transfer rate were calculated. The calculations were carried out using Phoenics Version 3.3 code. .

Tunnel-phase change heat exchanger

Abstract: A phase change beat exchanger and method for cooling a heat dissipating electronic component, such as an electronics package, transfers heat from the electronic component by way of a heatsink including a base in heat conducting relation with the electronic component and fins arranged in heat conducting relation at one end thereof with the base. The efficiency of the fins is increased by also transferring heat from the base to the fins at a location spaced from the one end thereof using a phase change fluid separated from the fins. A chamber containing the phase change fluid is defined between two telescoping tubes which extend peripherally about the fins to form a tunnel or duct through which a cooling fluid such as air can be flowed in contact with the fins. A further increase in cooling efficiency is obtained using an additional remote heat exchangers attached to the surface of the chamber.

Microchannel Heat Exchangers with Carbon Dioxide

Abstract: The objective of the present study was to determine the performance of CO{sub 2} microchannel evaporators and gas coolers in operational conditions representing those of residential heat pumps. A set of breadboard prototype microchannel evaporators and gas coolers was developed and tested. The refrigerant in the heat exchangers followed a counter cross-flow path with respect to the airflow direction. The test conditions corresponded to the typical operating conditions of residential heat pumps. In addition, a second set of commercial microchannel evaporators and gas coolers was tested for a less comprehensive range of operating conditions. The test results were reduced and a comprehensive data analysis, including comparison with the previous studies in this field, was performed. Capacity and pressure drop of the evaporator and gas cooler for the range of parameters studied were analyzed and are documented in this report. A gas cooler performance prediction model based on non-dimensional parameters was also developed and results are discussed as well. In addition, in the present study, experiments were conducted to evaluate capacities and pressure drops for sub-critical CO{sub 2} flow boiling and transcritical CO{sub 2} gas cooling in microchannel heat exchangers. An extensive review of the literature failed to indicate any previous systematic study in this area, suggesting a lack of fundamental understanding of the phenomena and a lack of comprehensive data that would quantify the performance potential of CO{sub 2} microchannel heat exchangers for the application at hand. All experimental tests were successfully conducted with an energy balance within {+-}3%. The only exceptions to this were experiments at very low saturation temperatures (-23 C), where energy balances were as high as 10%. In the case of evaporators, it was found that a lower saturation temperature (especially when moisture condensation occurs) improves the overall heat transfer coefficient significantly. However, under such conditions, air side pressure drop also increases when moisture condensation occurs. An increase in airflow rate also increases the overall heat transfer coefficient. Air side pressure drop mainly depends on airflow rate. For the gas cooler, a significant portion of the heat transfer occurred in the first heat exchanger module on the refrigerant inlet side. The temperature and pressure of CO{sub 2} significantly affect the heat transfer and fluid flow characteristics due to some important properties (such as specific heat, density, and viscosity). In the transcritical region, performance of CO{sub 2} strongly depends on the operating temperature and pressure. Semi-empirical models were developed for predictions of CO{sub 2} evaporator and gas cooler system capacities. The evaporator model introduced two new factors to account for the effects of air-side moisture condensate and refrigerant outlet superheat. The model agreed with the experimental results within {+-}13%. The gas cooler model, based on non-dimensional parameters, successfully predicted the experimental results within {+-}20%. Recommendations for future work on this project include redesigning headers and/or introducing flow mixers to avoid flow mal-distribution problems, devising new defrosting techniques, and improving numerical models. These recommendations are described in more detail at the end of this report.

Analysis of microchannel heat exchangers using FEM

Abstract: A finite element method is applied to evaluate the performance of microchannel heat exchangers that are used in electronic packaging. The present approach is validated against the CFD data available in the literature. A comparison of the predicted results with other available results obtained from different concepts shows that the present method is able to predict the surface temperature, the fluid temperature and thus the total thermal resistance of the microchannel heat sink satisfactorily. The present methodology has an added advantage in that non‐uniform surface heat flux distribution over the package base can also be analysed easily. The method used in the present analysis is an alternative to massive CFD calculations.

A numerical investigation of heat transfer enhancement in offset strip fin heat exchangers in self-sustained oscillatory flows

Abstract: Numerical analysis of the instantaneous flow and heat transfer has been carried out for offset strip fin geometries in self‐sustained oscillatory flow. The analysis is based on the two‐dimensional solution of the governing equations of the fluid flow and heat transfer with the aid of appropriate computational fluid dynamics methods. Unsteady calculations have been carried out. The obtained time‐dependent results are compared with previous numerical and experimental results in terms of mean values, as well as oscillation characteristics. The mechanisms of heat transfer enhancement are discussed and it has been shown that the fluctuating temperature and velocity second moments exhibit non‐zero values over the fins. The creation processes of the temperature and velocity fluctuations have been studied and the dissimilarity between these has been proved.

Extruded microchannel heat exchanger

Abstract: A fluid-to-fluid heat exchanger comprising a body having channels side-to-side and extending end-to-end, said channels being capable of directing fluids in alternate and opposite (counter-flow) directions in the body, the body being crimped adjacent to said ends so that the fluid is fully enclosed in said body. Fluid entrance and exit openings in the body so that the alternate channels within the body can enable the fluid to achieve the desired conclusion of the heat exchanger, and manifolds secured to said heat exchanger so that they communicate with alternate channels within the heat exchanger, and some channels are not of the same cross sectional size so that some channels can move more fluid in a given time and are smaller than other channels.

Corrugated fin with partial offset for a plate-type heat exchanger and corresponding plate-type heat exchanger

Abstract: In this corrugated fin, each corrugation leg ( 10 A, 10 B) has a notch ( 18 A, 18 B) on at least one edge ( 11 A) and over at least part of its height. Application to brazed-plate heat exchangers.