Showing papers on "Micro heat exchanger published in 2005"

Transient flow pattern based microscale boiling heat transfer mechanisms

Abstract: In our previous paper (J L Xu et al 2005 J. Micromech. Microeng. 15 362–76), it is identified that the transient flow patterns for microscale boiling heat transfer are repeated on the timescale of milliseconds. A full cycle could be subdivided into three substages: liquid refilling stage, bubble nucleation, growth and coalescence stage and transient annular flow stage. Five heat transfer mechanisms could be deduced from the transient flow patterns. This paper extends the above work and mainly focuses on the boiling heat transfer behavior, which was performed for 102 runs with the following data ranges: inlet pressures of 1–2 bar, inlet liquid temperatures of 24–45 °C, pressure drops of 10–100 kPa, mass fluxes of 64–600 kg m−2 s−1, heat fluxes of 150–480 kW m−2, exit vapor qualities of 0.07–1.15 and the boiling numbers of 0.69 × 10−3–5.046 × 10−3. The silicon wafer test section consists of ten triangular microchannels with the hydraulic diameter of 155.4 µm. Acetone is selected as the working fluid. The heat transfer coefficients were analyzed with the effects of the heat fluxes, the mass fluxes and the vapor mass qualities. We provide a link between the transient flow patterns and the heat transfer process. The boiling numbers can be used to characterize the microscale boiling heat transfer, which can display three distinct regions by dividing the boiling numbers into three subranges. The transient flow pattern based heat transfer mechanisms are very consistent with the heat transfer coefficient measurements with the effects of the heat fluxes, mass fluxes and vapor mass qualities. The transition boundaries among the three heat transfer regions are given.

Micro heat exchanger with thermally conductive porous network

Abstract: A micro heat exchanger system includes a first flow path and a second flow path transverse thereto for transferring thermal energy between a first fluid flowing through the first flow path and a second fluid flowing through the second flow path. The first flow path and the second flow path are filled with a thermally conductive porous network which incorporate unique structures, such as tubes, honeycomb, corrugated metal, reticulated foams, woven meshes or nonwoven mats or felts, engineered lattice structures, or a combination of these structures. In another embodiment, the thermally conductive porous network is coated with catalyst to provide an integrated heat exchanger and catalytic reactors.

Air-side heat transfer enhancement of a refrigerator evaporator using vortex generation

Abstract: In most domestic and commercial refrigeration systems, frost forms on the air-side surface of the air-to-refrigerant heat exchanger. Frost-tolerant designs typically employ a large fin spacing in order to delay the need for a defrost cycle. Unfortunately, this approach does not allow for a very high air-side heat transfer coefficient, and the performance of these heat exchangers is often air-side limited. Longitudinal vortex generation is a proven and effective technique for thinning the thermal boundary layer and enhancing heat transfer, but its efficacy in a frosting environment is essentially unknown. In this study, an array of delta-wing vortex generators is applied to a plain-fin-and-tube heat exchanger with a fin spacing of 8.5 mm. Heat transfer and pressure drop performance are measured to determine the effectiveness of the vortex generator under frosting conditions. For air-side Reynolds numbers between 500 and 1300, the air-side thermal resistance is reduced by 35–42% when vortex generation is used. Correspondingly, the heat transfer coefficient is observed to range from 33 to 53 W m−2 K−1 for the enhanced heat exchanger and from 18 to 26 W m−2 K−1 for the baseline heat exchanger.

Microscale Heat Transfer Fundamentals and Applications

Abstract: Preface. Single-Phase Forced Convection in Microchannels - State-of-the-Art Review Y. Yener et al. Measurements of Single-Phase Pressure Drop and Heat Transfer Coefficient in Micro and Minichannels A. Bontemps. Steady State and Periodic Heat Transfer in Micro Conduits M.D. Mikhailov et al. Flow Regimes in Microchannel Single-Phase Gaseous Fluid Flow Y. Bayazitoglu, S. Kakac. Microscale Heat Transfer at Low Temperatures R. Radebaugh. Convective Heat Transfer for Single-Phase Gases in Microchannel Slip Flow: Analytical Solutions Y. Bayazitogluet al. Microscale Heat Transfer Utilizing Microscale and Nanoscale Phenomena A. Yabe. Microfluidics in Lab-on-a-Chip: Models, Simulations and Experiments Dongquing Li. Transient Flow and Thermal Analysis in Microfluidics R.M. Cotta et al. From Nano to Micro to Macro Scales in Boiling V.K. Dhir et al. Flow Boiling in Minichannels A. Bontemps et al. Heat Removal Using Narrow Channels, Sprays and Microjets M. Fabbri et al. Boiling Heat Transfer in Minichannels V. Kuznetsov et al. Condensation Flow Mechanisms, Pressure Drop and Heat Transfer in Microchannels S. Garimella. Heat Transfer Characteristics of Silicon Film Irradiated by Pico to Femtosecond Lasers J. Sik Lee, S. Park. Microscale Evaporation Heat Transfer V.V. Kuznetsov, S.A. Safonov. Ultra-Thin Film Evaporation(UTF)-Application to Emerging Technologies in Cooling of Microelecronics M. Ohadi, J. Qi. Binary-Fluid Heat and Mass Transfer in Microchannel Geometries for Miniaturized Thermally Activated Absorption Heat Pumps S. Garimella. Heterogeneous Crystallization of Amorphous Silicon Accelerated by External Force Field: Molecular Dynamics Study J. Sik Lee, S. Park. Hierarchical Modeling of Thermal Transport from Nano-to-Macroscales C.H. Amon et al. Evaporative Heat Transfer on Horizontal Porous Tube L. Vasiliev et al. Micro and Miniature Heat Pipes L.L. Vasiliev. Role of Microscale Heat Transfer in Understanding Flow Boiling Heat Transfer and Its Enhancement K. Sefiane, V.V. Wadekar. Heat Transfer Issues in Cryogenic Catheters R. Radebaugh. Sorption Heat Pipe - A New Device for Thermal Control and Active Cooling L.L. Vasiliev, L. Vasiliev, Jr. Thermal Management of Harsh-Environmental Electronics M. Ohadi, J. Qi. Index.

Flow and Heat Transfer in Narrow Channels with Corrugated Walls: A CFD Code Application

Abstract: In an effort to obtain information on the local flow structure inside compact heat exchangers made of corrugated plates, a commercial CFD code ( CFX ® ) is employed to simulate the flow through an element of this type of equipment. For simplicity, the channel used for the simulation is formed by only one corrugated plate, while the other plate is flat. A two-equation turbulence model ( SST ) is used for the calculations and, in addition to isothermal flow, heat transfer simulations are conducted for a Reynolds number range (400–1400), for the case of hot water (60°C) in contact with a constant-temperature wall (20°C). The results, presented in terms of friction factor, wall shear stress, wall heat flux and local Nusselt numbers, are consistent with the description of the fluid motion inside similar conduits by other investigators. The calculated mean heat transfer coefficients and friction factors are found to be in reasonable agreement with the limited published experimental data.

Characterizing heat transfer coefficients for heat exchangers in standing wave thermoacoustic coolers

Abstract: The thermal performance of heat exchangers within a thermoacoustic cooler was investigated. Experimental procedures and calculation methods to evaluate the oscillating flow heat transfer coefficients were developed. Dimensionless heat transfer coefficients, or Colburn-j factors, were estimated based on the oscillating-flow variables and compared with results from steady-flow measurements. The results were also compared with heat transfer coefficients predicted from a boundary-layer conduction model, and methods that utilize steady-flow correlations with Reynolds numbers that characterize oscillating flow conditions. Although the boundary layer model is commonly employed for thermoacoustic calculations, it did not accurately predict heat transfer coefficients and the influence of Reynolds number on heat transfer performance. However, accurate predictions were obtained using a steady-flow correlation and a modified Reynolds number that accounts for the oscillating flow field, assuming a half-cycle Reynolds ...

Microchannel heat exchanger with micro-encapsulated phase change material for high flux cooling

Abstract: A microchannel cooler containing a slurry having a particulate liquid/solid phase change material is provided balancing the interdependent factors of microencapsulated particle size with microchannel size and shape and flow conditions for the removal of high heat flux with low space and low power requirements.

Helical coil-on-tube heat exchanger

Abstract: A coil on tube heat exchanger is provided that uses multiple parallel helical channels to limit liquid pressure losses while providing similar performance and production times to previous coil and tube designs. Two or more channels are wrapped together around a tube in a helical fashion, permitting the heat exchanger to be used in a counter-flow, or contra-flow, implementation. The system preferably includes a header, or manifold, to connect two or more of the channels together at the beginning and/or end of the tube. However, each individual channel may be connected to a separate load and kept independent. The plurality of channels can be implemented within a single tube or via multiple tubes, or a combination thereof. Embodiments of the present invention provide reduced pressure loss, higher performance and are generally faster to manufacture than prior heat exchangers.

Measurement and Modeling of Condensation Heat Transfer Coefficients in Circular Microchannels

Abstract: A model for predicting heat transfer during condensation of refrigerant R134a in horizontal microchannels is presented. The thermal amplification technique is used to measure condensation heat transfer coefficients accurately over small increments of refrigerant quality across the vapor-liquid dome. A combination of a high flow rate closed loop primary coolant and a low flow rate open loop secondary coolant ensures the accurate measurement of the small heat duties in these microchannels and the deduction of condensation heat transfer coefficients from measured UA values. Measurement were conducted for three circular microchannels (0.5 < Dh < 1.5 mm) over the mass flux range 150 < G < 750 kg/m2 -s. Results from previous work by the authors on condensation flow mechanisms in microchannel geometries were used to interpret the results based on the applicable flow regimes. The heat transfer model is based on the approach originally developed by Traviss et al. (1973) and Moser et al. (1998). The multiple-flow-regime model of Garimella et al. (2005) for predicting condensation pressure drops in microchannels is used to predict the pertinent interfacial shear stresses required in this heat transfer model. The resulting heat transfer model predicts 86% of the data within ±20%.Copyright © 2005 by ASME

Effects of Distributor Configuration on Flow Maldistribution in Plate-Fin Heat Exchangers

Abstract: In this paper, an original concept of a design that adds a complementary fluid cavity in the distributor is presented. The experimental investigation of the effects of distributor configuration parameter on the fluid flow maldistribution in the plate-fin heat exchanger is completed. The correlation of the dimensionless flow maldistribution parameter and the Reynolds number is obtained under different distributor configuration parameters. The experimental studies prove that the performance of flow distribution in heat exchangers can be effectively improved by the optimum design of the distributor's configuration parameter. The ratio of the maximum velocity and the minimum velocity in the channels of the plate-fin heat exchanger can drop from 2.57–3.66 to 2.08–2.81 for various Reynolds numbers. The conclusions are of great significance on the optimum structure design of the plate-fin heat exchangers and can effectively improve the performance of the heat exchangers.

Effect of heat transfer on the performance of two-stage semiconductor thermoelectric refrigerators

Abstract: A model of two-stage semiconductor thermoelectric refrigerators with external heat transfer is proposed. The performance of the refrigerator obeying Newton’s heat transfer law is analyzed using the combination of finite-time thermodynamics and nonequilibrium thermodynamics. Two analytical formulas for cooling load versus working electrical current, and the coefficient of performance (COP) versus working electrical current, are derived. For a fixed total heat transfer surface area of two heat exchangers, the ratio of the heat transfer surface area of the high-temperature-side heat exchanger to the total heat transfer surface area of the heat exchangers is optimized to maximize the cooling load and the COP of the thermoelectric refrigerator. For a fixed total number of thermoelectric elements, the ratio of the number of thermoelectric elements of the top stage to the total number of thermoelectric elements is also optimized to maximize both the cooling load and the COP of the thermoelectric refrigerator. The effects of design factors on the performance are analyzed.