Showing papers on "Micro heat exchanger published in 2002"

Air-side thermal hydraulic performance of multi-louvered fin aluminum heat exchangers

Abstract: An experimental study on the air-side heat transfer and pressure drop characteristics for multi-louvered fin and flat tube heat exchangers has been performed. For 45 heat exchangers with different louver angles (15–29°), fin pitches (1.0, 1.2, 1.4 mm) and flow depths (16, 20, 24 mm), a series of tests were conducted for the air-side Reynolds numbers of 100–600, at a constant tube-side water flow rate of 0.32 m3/h. The inlet temperatures of the air and water for heat exchangers were 21 and 45°C, respectively. The air-side thermal performance data were analyzed using effectiveness-NTU method for cross-flow heat exchanger with both fluid unmixed conditions. The heat transfer coefficient and pressure drop data for heat exchangers with different geometrical configurations were reported in terms of Colburn j-factor and Fanning friction factor f, as functions of Reynolds number based on louver pitch. The general correlations for j and f factors are developed and compared to other correlations. The f correlation indicates that the flow depth is one of the important parameters for the pressure drop.

Heat Transfer in Single and Multiphase Systems

Abstract: INTRODUCTION Vector and Tensor Notations Fundamental Concepts and Definitions Eulerian and Lagrangian Descriptions Properties of a System Conductive Heat Transfer Convective Heat Transfer Radiation Heat Transfer Phase Change Heat Transfer Conservation of Energy Problems CONDUCTION HEAT TRANSFER Introduction One-Dimensional Heat Conduction Thermal and Contact Resistances Fins and Extended Surfaces Multidimensional Heat Conduction Graphical Solution Methods Analytical Methods Transient Heat Conduction Combined Transient and Spatial Effects References Problems CONVECTIVE HEAT TRANSFER Introduction Convection Governing Equations Velocity and Thermal Boundary Layers External Forced Convection Internal Forced Convection Free Convection Second Law of Thermodynamics Turbulence Modelling References Problems RADIATIVE HEAT TRANSFER Introduction Fundamental Processes and Equations Radiation Exchange Between Surfaces Thermal Radiation in Enclosures with Diffuse Gray Surfaces Solar Energy References Problems PHASE CHANGE HEAT TRANSFER Introduction Processes of Phase Change Mixture and Two-Fluid Formulations Interface Tracking References Problems GAS (VAPOR) - LIQUID SYSTEMS Introduction Boiling Heat Transfer Condensation Heat Transfer Devices with Vapor - Liquid Phase Change References Problems GAS - SOLID (PARTICLE) SYSTEMS Introduction Classification of Gas - Solid Flows Dynamics of Gas - Solid Flows Fluidized Beds References Problems LIQUID - SOLID SYSTEMS Introduction One-Dimensional Solidification and Melting Phase Change with Convection Phase Change with Coupled Heat and Mass Transfer Problems in Other Geometries Multi-Dimensional Solidification and Melting Dynamics of Liquid - Solid Flows Applications References Problems GAS -LIQUID - SOLID SYSTEMS Introduction Droplet Flows with Phase Change Gas Flows with Solidification and Melting Chemically Reacting Systems Multiphase Byproducts of Reacting Flows References Problems HEAT EXCHANGERS Introduction Tubular Heat Exchangers Cross-Flow and Shell-and-Tube Heat Exchangers Effectiveness - NTU Method of Analysis Condensers and Evaporators References Problems COMPUTATIONAL HEAT TRANSFER Finite Difference Methods Weighted Residual Methods Finite Element Method Hybrid Methods Numerical Methods for Other Applications Accuracy and Efficiency Improvements References Problems APPENDICES INDEX

A pistonless Stirling cooler

Abstract: We demonstrate a prototype acoustic cooler that uses Stirling cycles executed by a traveling wave with high acoustic impedance thermoacoustically induced in a looped tube. The tube has no moving parts, only a pair of stacks sandwiched between two heat exchangers: one amplifies the acoustic power and the amplified wave supplies the driving energy to pump heat directly within the second stack. Because it uses extremely simple hardware consisting of a few parts, the cooling device is potentially a powerful tool for applications such as conventional cooling systems.

Air-side performance of brazed aluminum heat exchangers under dehumidifying conditions

Abstract: An experimental study for air-side thermal-hydraulic performance of brazed aluminum heat exchangers under dehumidifying conditions has been performed. For 30 samples of louvered fin heat exchangers with different geometrical parameters, the heat transfer and pressure drop characteristics for wet surface were evaluated. The test was conducted for air-side Reynolds number in the range of 80–300 and tube-side water flow rate of 320 kg/h. The dry- and wet-bulb temperatures of the inlet air for heat exchangers were 27 and 19 °C, respectively and the inlet water temperature was 6 °C. The air-side thermal performance data for cooling and dehumidifying conditions were analyzed using effectiveness- NTU method for cross-flow heat exchanger with both fluids unmixed. The test results are reported, compared with those for the dry surface heat exchangers, in terms of sensible j factor and friction factor f , as functions of Reynolds number based on louver pitch. The correlations for j and f factors are developed within rms errors of ±16.9 and ±13.6%, respectively.

Fabrication, modeling, and testing of micro-cross-flow heat exchangers

Abstract: Planar micro-cross-flow heat exchangers, similar in concept to most automobile radiators, have been fabricated using two different processes. A process that was previously reported (Harris et al., 2000) to fabricate a polymer heat exchanger involved embossing two identical polymer parts using the LIGA process. Then the two parts were aligned and bonded together. In this paper, a process is described to fabricate a nickel micro-cross-flow heat exchanger by embossing a sacrificial polymer mandrel using a LIGA-fabricated mold insert. The mandrel is coated with nickel (using either electroplating or electroless plating), then the sacrificial mandrel is dissolved. Experimental results are reported for both the polymer and nickel heat exchangers to determine the rates of heat transfer between the in-plane liquid (water) and the through-plane gas (air). Pressure drops of both fluid streams were also measured. The experimental results compare favorably with a modified version of the analytical model that was described previously. The fabricated heat exchangers have values of heat transfer/volume that are more than five times higher than conventional scale counterparts (with characteristic dimensions at least one order of magnitude larger than those reported here) and values of heat transfer/mass that are 50% greater than their conventional scale counterparts.

Flexible microchannel heat exchanger

Abstract: A flexible mesoscopic heat exchanger is provided by the invention. The heat exchanger of the invention includes uniform microchannels for fluid flow. Separate header and channel layers include microchannels for fluid flow and heat exchange. A layered structure with channels aligned in multiple orientations in the layers permits the use of a flexible material without channel sagging and provides uniform flows. In a preferred embodiment, layers are heat sealed, e.g., by a preferred lamination fabrication process.

Method of action of the pulsating heat pipe, its construction and the devices on its base

Abstract: By construction of heat pipes with two phase heat carrying fluid, an efficient heat transfer system is available. The heating section is used to create, within the heat pipe, build up of vapor pressure which causes the heat carrying fluid to move which then allows for dissipations of the heat into the cooling section. Additions to the system allow for storage of kinetic energy to enhance or regulate the flow, and by introduction of heating elements or porous surfaces, the mechanic can be improved or regulated.

High performance cold plate

Abstract: A cold plate assembly for cooling a heat source is disclosed. The cold plate assembly includes a thermally conductive base having an inner surface for thermally interfacing with the heat source. Machining, casting, or molding a contour to interface with the heat source forms the base. Heat pipe thermal plane or individual heat pipes are selected to conform to the thermal cooling capacity of the cold plate assembly. One or two compact heat exchangers are selected to remove the heat conducted through the base to the cooling liquid is used in the compact heat exchanger. The three modules are bonded together to form the cold plate assembly.

Stacked low profile cooling system and method for making same

Abstract: A stacked array of low profile heat pipes each with a plurality of micro tubes extended therethrough. The stacked low profile heat pipes are placed into thermal connection with heat producing components. A heat transfer fluid is contained in the micro tubes of the low profile heat pipes and removes the heat from the heat producing components.

Single-Phase Pressure Drop Measurements in a Microchannel Heat Exchanger

Abstract: Single-phase pressure drops inside the complex headers and parallel circuits of a microchannel heat exchanger were measured systematically, for the purpose of identifying and quantifying internal manufacturing defects. Results confirmed the Moody chart to be applicable for the submillimeter channel flows. Based on nonintrusive testing followed by destructive evaluation, two types of manufacturing defects were quantified: variation of microchannel port diameters and port blockage by brazing flux. A pressure drop model for the whole heat exchanger was developed, and predicted the pressure and mass flow rate distribution inside the heat exchanger, providing valuable insights for selecting microchannel tube dimensions and heat exchanger circuiting. Most important, simple nitrogen flow tests can now be used to help control the quality of the brazing process, by detecting the presence of blocked or deformed ports in the finished product.

Numerical optimisation of a two-stage ejector refrigeration plant

Abstract: Jet-refrigeration cycles seem to provide an interesting solution to the increasing interest in environment protection and the need for energy saving due to their low plant costs, reliability and possibility to use water as operating fluid. A steam/steam ejector cycle refrigerator is investigated introducing a two-stage ejector with annular primary at the second stage. The steady_state refrigerator, exchanging heat with the water streams at inlet fixed temperatures at the three shell and tube heat exchangers, evaporator, condenser and generator, is considered as an open system. Heat transfer irreversibilities in the heat exchangers and external friction losses in the water streams are considered, ignoring the internal pressure drop of the vapor. A simulation program numerically searches the maximum COP at given external inlet fluid temperatures as a function of mass flows, dimensions and temperature differences in the heat exchangers. The code gives the ejector and heat exchangers design parameters.

Absorption heat-transfer system

Abstract: A waste heat source (100) is used to heat a high temperature heat transfer fluid which is used to heat an absorption heat transfer machine (10) having a generator (20), an absorber (30), a condenser (40), and an evaporator (50) operatively connected together. The high temperature heat transfer fluid can also be used to heat a load (190) such as a room space or a process. The waste heat source (100) can also be used to heat an intermediate heat transfer fluid, which can be used to heat a second load (175) such as a space, a process, or an absorption heat transfer machine. Novel flow control devices (70, 60) for controlling the flow of weak sluution from generator (20) to absorber (30) or of refrigerant from condenser (40) to evaporator (50), respectively, are also described.

Evolution of Microchannel Flow Passages: Thermohydraulic Performance and Fabrication Technology

Abstract: This paper provides a roadmap of development in the thermal and fabrication aspects of microchannels as applied in the microelectronics and other high heat-flux cooling applications. Microchannels are defined as flow passages that have hydraulic diameters in the range of 10 to 200 micrometers. The impetus for microchannel research was provided by the pioneering work of Tuckerman and Pease [1] at Stanford University in the early eighties. Since that time, this technology has received considerable attention in microelectronics and other major application areas, such as fuel cell systems and advanced heat sink designs. After reviewing the advancement in heat transfer technology from a historical perspective, advantages of using microchannels in high heat flux cooling applications is discussed, and research done on various aspects of microchannel heat exchanger performance is reviewed. Singlephase performance for liquids is expected to be still describable by the conventional equations; however the gas flow may be influenced by the rarefaction effects. Two-phase flow is another topic that is still under active research. The evolution of research into microchannel heat sinks has paralleled the advancements made in microfabrication technology. The earliest microchannels were built using anisotropic wet chemical etching techniques based on alkali solutions. While this method has been exploited successfully, it does impose certain restrictions on silicon wafer type and geometry. Recently, anisotropic dry etching processes have been developed that circumvent these restrictions. In addition, dry etching methods can be significantly faster and, from a manufacturing standpoint, create fewer contamination and waste treatment problems. Advances in fabrication technology will continue to fuel improvements in microchannel heat sink performance and cost for the foreseeable future. Some fabrication areas that may spur advances include new materials, high aspect ratio patterning techniques other than dry etching, active fluid flow elements, and micromolding NOMENCLATURE

Hydrodynamic Studies on Heat Exchangers with Helical Baffles

Abstract: The flow field in shell-and-tube heat exchangers with helical baffles was measured using laser Doppler anemometry (LDA). The influence on the velocity distribution, impulsive velocity by helix inclination angle, and flow rate was investigated. The influence on heat exchanging capability and flow resistance on velocity distribution was also investigated. The dimensions of the heat exchanger shell used in these experiments were 200 2 6 2 3,000 mm (inner diameter 2 wall thickness 2 length). The heat exchanger was made of organic glass and the tube bundle consisted of 52 tubes with external diameter of 15 mm. Six different inclination angles were designed in double-helix style: 30°, 35°, 40°, 42°, 45°, and 50°. The working flow medium under normal temperature was service water. Generally, the linear velocity and impulsive velocity will increase with decreasing helix inclination angle, which promotes the heat exchanging capability. With flow volume increasing, the velocity distribution along the diameter incre...

Heat Convection in Micro Ducts

Abstract: Foreword. Preface. Acknowledgements. 1. Introduction. 2. Fundamentals of Microscale Convective Heat Transfer. 3. Scaling, Similarity and Dimensionless Parameters in Convective Heat Transfer. 4. Fabrication of Thermal Microsystems. 5. Thermometry Techniques for Microscale Heat Convection Measurements. 6. Steady, Single-Phase Heat Convection in Micro Ducts. 7. Steady, Forced Convection Boiling in Micro Ducts. 8. Unsteady Convective Heat Transfer in Micro Ducts. 9. Micro Heat Pipes. References. Index.

The Influence of Heat Transfer Irreversibilities on the Optimal Performance of Diabatic Distillation Columns

Abstract: A distillation column with the possibility of heat exchange on every tray (a fully diabatic column) is optimized in the sense of minimizing its total entropy production. This entropy production counts the interior losses due to heat and mass flow as well as the entropy generated in the heat exchangers. It is observed that the optimal heating distribution, i.e. the heat exchange required on each tray, is essentially the same for all trays in the stripping and rectification sections, respectively. This makes a column design with consecutive interior heat exchanger and only one exterior supply for each of the two sections very appealing. The result is only slightly dependent on the heat transfer law considered. In the limit of an infinite number of trays even this column with resistance to transfer of heat becomes reversible.

Numerical prediction of heat transfer in a channel with a built-in oval tube and two different shaped vortex generators

Abstract: Existing air-cooled condensers in geothermal power plants use fin-tube heat exchangers with circular tubes. The size of the air-cooled condensers is huge, and often the cost of the condensers is about one third of the plant cost. An effort is being made to reduce the size of the air-cooled condensers by enhancing the transport coefficients on the heat exchange surfaces. Improvement over the common fin-tube heat exchangers can be brought about by a strategic modification. First, the oval tubes may replace the circular tubes. Second, the delta-winglet type vortex generators can be added on the flat surfaces to improve the heat transfer even further. Finally, a rectangular winglet pair can be deployed instead of a delta winglet pair and the performance of two different shaped vortex generators can be compared. A computational study has been undertaken to show the promise of above-mentioned combinations on a representative element of a heat exchanger.