Showing papers on "Micro heat exchanger published in 1991"

Thermoelectric device with recuperative heat exchangers

Abstract: Thermoelectric heat pumps using recuperative heat exchange are described. These devices use sets of thermocouples (thermoelectric couples) arranged side-by-side to form a plate. The plate is positioned in a fluid-containing vessel and heat exchanging fluid is flowed down one side of the plate and up the other side. In these devices the heat flow, and thus the driving thermal gradient on each thermoelectric couple in the device, is in a direction from one side of the plate to the other side, i.e., other than the direction of the device's working thermal gradient, which is the direction of the flow of fluid. Generally these two directions (driving gradient on the thermoelectric couples and fluid flow-working thermal gradient) are essentially orthogonal to each other.

Transient Two-Dimensional Compressible Analysis for High-Temperature Heat Pipes with Pulsed Heat Input

Abstract: A complete mathematical model for transient two-dimensional heat pipes is presented. The numerical results for both simulated compressible vapor flow with high Mach numbers and the vapor flow of a high-temperature heat pipe are compared with the experimental data in the literature. The transient responses of heat pipes to a pulsed heat input are also investigated. It is very important to include the porous wick and the wall in the numerical calculations for the transient analysis of heat pipes and to treat the entire heat pipe as a single system rather than to analyze the vapor flow alone.

Investigation of the transient characteristics of a micro heat pipe

Abstract: An analytical investigation was conducted to determine the potential advantages of incorporating very small (100 jim) "micro" heat pipes directly into semiconductor devices. As a result of this investigation, a transient numerical model capable of predicting the thermal behavior of these micro heat pipes during startup or variation in the evaporator thermal load was developed. This numerical model was used to identify, evaluate, and better understand the phenomena that govern the transient behavior of micro heat pipes as a function of the physical shape, the properties of the working fluid, and the principal dimensions. The modeling results were compared with the steady-state results from an earlier experimental investigation and were shown to accurately predict the steady state dry out limit for two different test pipes. Using the verified numerical model, the parameters that affect the axial heat transport capacity were evaluated. The results of this evaluation indicate that in micro heat pipes reverse liquid flow occurs in the liquid arteries during startup and/or rapid transients. In addition, the wetting angle was found to be one of the most important factors contributing to the transport capacity. Nomenclature A = cross-sectional area Cp = specific heat d = distance j = evaporation M = molecular weight m = mass flow rate P = pressure Q = heat transfer q = heat transfer rate R = universal gas constant r = radius of curvature T = temperature t = time V = velocity W = wetted perimeter x — length or distance y = distance

Condensation heat transfer inside vertical and inclined thermosyphons

Abstract: In recent years, heat-pipe heat exchangers have played a significant role in waste heat recovery. In some cases, the heat-pipe heat exchangers are required to be inclined to gravity as several experimental investigations have reported. Similarly Chato studied laminar condensation inside inclined tubes. However, no uniform conclusion on optimum inclination angle can be made due to the differences among the operating conditions. In this paper, both theoretical and experimental studies on vertical and inclined thermosyphons have been carried out. A semi-empirical correlation is presented for the purpose of designing heat pipe heat exchangers.

Transient Multidimensional Analysis of Nonconventional Heat Pipes With Uniform and Nonuniform Heat Distributions

Abstract: A numerical analysis of transient heat pipe performance including nonconventional heat pipes with nonuniform heat distributions is presented. A body-fitted grid system was applied to a three-dimensional wall and wick model, which was coupled with a transient compressible quasi-one-dimensional vapor flow model. The numerical results were first compared with experimental data from cylindrical heat pipes with good agreement. Numerical calculations were then made for a leading edge heat pipe with localized high heat fluxes. Performance characteristics different from conventional heat pipes are illustrated and some operating limits concerning heat pipe design are discussed.

Modular heat exchanger having delayed heat transfer capability

Abstract: A heat exchanger has a multiplicity of modules with an outer shell which is open at the top and the bottom and is joined with other modules in a stacked array. The heat exchanger has an insulating partition which is attached to the inside surface of the shell and divides the shell into two or more flow channels. The insulating partition has a hole which connects the flow channels defined by the shell and baffle. The hole is occupied by a heat-conductive heat storage reservoir, so that heat flow from one channel to another is through the heat storage reservoir. In this way, heat may pass from one flow channel into the storage reservoir and at a latter time be exchanged with a fluid flowing in another flow channel. Heat exchange modules have multiple flow channels and the flow channels preferably have heat conducting fins for increasing the heat transfer between the flow channels and the heat storage reservoir.

Characteristics of a gravity-assisted heat pipe-based heat exchanger

Abstract: An experimental study of the performance of an air-to-air thermosyphon-based heat exchanger utilizing R-22 as the working fluid has been carried out to investigate its behavior under different operating conditions. A test installation has been developed to model a variety of HVAC real life applications. The results reported in this article describe the influence of various parameters such as: supply and exhaust air stream mass flow rates, stream temperatures and exhaust stream moisture content on the effectiveness of the heat exchangers. Heat exchanger heat flow hysteresis has been recognized. Some optimization criteria are presented.

A compact dilution refrigerator with vertical heat exchangers for operation to 2 mK

Abstract: We describe a compactly designed dilution refrigerator with closely packed, vertical heat exchangers. The refrigerator reaches a temperature of 2 mK and is easily constructed, since the sintered heat exchangers are straight units. Vibrating wire resonators are employed in the mixing chamber as diagnostic tools, which may act as both thermometers and phase-boundary level indicators. There is a design problem in the vertical arrangement, namely, the sumps on the concentrated phase side that can slowly fill with dilute phase and degrade the performance. The problem is solved by draining the superfluid4He component in any collected dilute phase through superleaks into the mixing chamber.

Optimal design of crossflow heat exchangers

Abstract: The design of plate-fin and tube-fin crossflow heat exchangers is discussed. The transfer surface area of crossflow heat exchangers is used ineffectively because of the nonuniform distribution of the heat transfer across the volume of the exchanger. The optimal distribution of the transfer surface area for maximum heat exchanger effectiveness and constant total surface area is determined. It is found that a Dirac delta distribution of the transfer surface aligned along the diagonal of the crossflow exchanger gives the best performance; equal to that of a counterflow device. Design guidelines for optimal area allocation within crossflow heat exchangers are established. Compared to conventional designs, designs following these guidelines may lead to either a higher exchanger effectiveness for equal pressure drops and surface area, reduced pressure drops for equal exchanger effectiveness, or reduced weight and a near cubic form of the exchanger core for equal pressure drops and effectiveness.

Influence of scaling on the performance of shell-and-tube heat exchangers

Abstract: Fouling in shell-and-tube heat exchangers was modeled by combining Hasson's ionic diffusion model for scaling from CaCO3 solutions with a model for predicting the temperature distribution developed by Gaddis and Schlunder. Using the computed results, clean heat exchanger design rules were tested for fouling conditions. The effects of fouling on the efficiency of heat exchanger configurations were determined.

Large capacity re-entrant groove heat pipe

Abstract: A re-entrant groove heat pipe provides capillary channel fin surfaces which are smooth, continuous, and offer a large radius of curvature upon which an extended thin film of working liquid may develop. Superior formation of thin films of the working fluid allows heat to be conducted more readily between the surface of the heat pipe and the surface of the fluid where evaporation and condensation takes place.

Study on the heat transfer of heat exchangers for the Stirling engine. I : Heat transfer in a heated tube under the periodically reversing flow condition

Abstract: Heat-transfer characteristics in heated tubes under periodically reversing low conditions have been investigated experimentally using a test apparatus that simulates the heat exchangers for the actual Stirling engine. The experimental correlation considering the influence of the piston phase-angle difference for the heat-transfer coefficient has been induced by the use of the working gas velocity evaluated from the Schmidt cycle model which is one of the ideal Stirling cycles

Centrifugal heat pipe vapor absorption heat pump

Abstract: A heat pipe is disclosed for use in a vapor-absorption heat pump to provide high efficiency heat transfer between first and second fluids in contact with opposing first and second faces of the heat pipe. Although preferably disk-shaped and adapted for rotation for use in compact heat pumps, the heat pipe may be used in heat pump applications requiring other shapes, and may be either fixed or moving. In a further aspect of the invention, a vapor-absorption heat pump is provided incorporating at least one such heat pipe in at least one component thereof.

Method and apparatus for in situ testing of heat exchangers

Abstract: Using the method of this invention, the performance of a heat exchanger is determined by measuring the heat transfer capabilities of an individual tube. A relatively small reservoir of service fluid is connected to the inlet and outlet ports of a tube. The reservoir is provided with a heater or chiller and the service fluid is circulated through the tube. When a steady state is reached, the heat transfer characteristics of the tube are measured using known mathematical relationships.

Thermal Coupling in Laminar Flow Double-Pipe Heat Exchangers

Abstract: Thermal interaction between the streams of laminar flow double-pipe heat exchangers is investigated theoretically by accounting for axial conduction along the wall separating the fluids. In a countercurrent arrangement, thermal coupling is demonstrated to have a definite influence on all the more important heat transfer parameters, such as the wall temperature, the heat flux density, the local entropy production rate, and the Nusselt number distributions. The overall performance of the device is considered under a second law point of view, and a complete parametric study is carried out.

Two-phase flow heat exchange

Abstract: A heat exchange process and apparatus are especially suited for use in heat recovery applications, especially in a run-around heat exchanger. The run-around heat recovery system has two liquid to gas heat echangers (10, 12), one for heat transfer with stale air being exhausted and one for heat transfer with fresh air being taken in. The two heat exchangers (10, 12) are linked with a common coupling liquid so that heat may be extracted from one of the air flows and added to the other. To improve the heat transfer process, a gas is injected into the liquid just before it enters one or both of the heat exchangers. The resultant mixed phase flow produces a much higher heat transfer coefficient in the heat exchangers (10, 12), yielding a significant improvement in the system performance.

Development of the Single Graded Groove high-performance heat pipe

Abstract: This paper describes the development of a new nonarterial heat pipe with a nominal transport capability of 100,000 W in. Data are presented for one-g transport capability as a function of tilt and working fluid quantity. The transport capability agrees well with theoretical predictions. The LMSC Graded Groove Heat Pipe exhibits the high throughput and excellent heat transfer characteristics of earlier arterial designs such as the LMSC Tapered Artery Heat Pipe. At the same time, it suffers none of the priming difficulties associated with the arterial designs.

Heat transfer from tubes in mist flows

Abstract: The present work has been undertaken to pursue the high performance of mist-cooled heat exchangers, Hrrt, mist-cooling experiments are conducted for staggered tubes having a fine surface structure in a vertical crossflow, and mist-cooling heat transfer is discussed including the effects of mutual interferences among the tubes. Next, mist-cooling heat transfer is analyzed for an arbitrary configuration of tubes, and design optimization for mist-cooled heat exchangers is developed upon applying the performance-evaluation criteria method of Bergles.

Heat transfer in the flow of a single-phase and boiling coolant in a channel with a porous insert

Abstract: The results of an experimental investigation of heat transfer, hydraulic resistance, and critical heat fluxes for the flow of coolant in a channel with a square cross section and porcelain inserts consisting of highly porous permeable cellular materials, are presented.

Mechanical design of heat exchangers and pressure vessel components

Abstract: (1991). MECHANICAL DESIGN OF HEAT EXCHANGERS AND PRESSURE VESSEL COMPONENTS. Drying Technology: Vol. 9, No. 5, pp. 1333-1335.

On the Performance of High-N/tu Heat Exchangers with Variable Heat Capacity of the Working Fluid

Abstract: The common methods of heat exchanger design are based on the assumption that the heat capacity of each fluid stream remains constant throughout the exchanger. Heat exchangers with variable specific heat, however, can be analyzed only by either numerical techniques or one of several approximate methods. While these approximate methods are generally satisfactory, they fail when fluid heat capacities change in such a way that the ratio of heat capacity rates changes from a value less than unity to one greater than unity along the exchanger. Such a situation results in a “pinch point,” or point of lowest temperature approach, inside the exchanger rather than at one of the ends. Complete information on possible pinch points is necessary for rating and sizing of these exchangers, particularly in high-N tu units. In this article a mathematical model is given to predict the existence and location of pinch points without computing complete temperature profiles. A step-by-step procedure is given for comput...

Heat exchanger apparatus, particularly for hybrid heat pumps operated with non-azeotropic work fluids

Abstract: Heat exchanger apparatus which includes a substantially horizontal countercurrent heat exchanger of the shell-and-tube type, particularly for hybrid heat pumps operated with non-azeotropic work fluids. The apparatus further includes fluid distributor with fluid outlets the number of which corresponds to the number of the heat exchanger tubes of the heat exchanger is provided upstream the heat exchanger, the heat exchanger tubes of which are connected each to one fluid outlet of the fluid distributor.