Showing papers on "Micro heat exchanger published in 2007"

Transport Phenomena in Micro Process Engineering

Abstract: Micro Process Engineering - An Interdisciplinary Approach: Introduction and Motivation.- Orientation of Micro Process Engineering.- The Role of Transport Processes.- Some Requirements for Successful Microstructures.- Scaling dimensions.- Actual Applications.- Barriers and Challenges. Fundamental Balances and Transport Processes: Introduction.- Unit Operations and Process Design.- Balances and Transport Equations.- Modeling, Calculation Methods, and Simulation.-Future Directions of Micro Process Engineering Research. Momentum Transfer: Momentum Transfer of Single-Phase Flow.- Convective Fluid Dynamics in Microchannels.- Multiphase Flow. Heat Transfer and Micro Heat Exchangers: Heat Transfer Fundamentals.- Microfluidic Networks for Heat Exchange.- Micro Heat Exchanger Devices. Diffusion, Mixing, and Mass Transfer Equipment: Mixing Processes and Their Characterization.- Diffuse Mass Transport and Concentration Distribution in Fluids.- Convective Mass Transport.- Characteristics of Convective Laminar Micromixers.- Mixing and Chaotic Advection.- Design and Fabrication of Silicon Micromixers.- High Throughput Mixing Devices with Microchannels. Chemical Reactions and Reactive Precipitation: Chemical Reactor Engineering.- Wall Mass Transfer and Surface Reactions in Multifluidic Systems.- Design Criteria for Microchannel Reactors.- Microreactor for Aerosol Generation.- Mixing and Defined Precipitation in Liquid Phase. Coupled Transport Processes: Thermodynamics of Irreversible Processes.- Thermoelectric Energy Conversion.- Electroosmotic and Electrokinetic Effects.- Thermodiffusion.- Pressure Diffusion. Conclusion and Final Remarks: Bibliography, Index

An experimental study of shell-and-tube heat exchangers with continuous helical baffles

Abstract: Two shell-and-tube heat exchangers (STHXs) using continuous helical baffles instead of segmental baffles used in conventional STHXs were proposed, designed, and tested in this study. The two proposed STHXs have the same tube bundle but different shell configurations. The flow pattern in the shell side of the heat exchanger with continuous helical baffles was forced to be rotational and helical due to the geometry of the continuous helical baffles, which results in a significant increase in heat transfer coefficient per unit pressure drop in the heat exchanger. Properly designed continuous helical baffles can reduce fouling in the shell side and prevent the flow-induced vibration as well. The performance of the proposed STHXs was studied experimentally in this work. The heat transfer coefficient and pressure drop in the new STHXs were compared with those in the STHX with segmental baffles. The results indicate that the use of continuous helical baffles results in nearly 10% increase in heat transfer coefficient compared with that of conventional segmental baffles for the same shell-side pressure drop. Based on the experimental data, the nondimensional correlations for heat transfer coefficient and pressure drop were developed for the proposed continuous helical baffle heat exchangers with different shell configurations, which might be useful for industrial applications and further study of continuous helical baffle heat exchangers. This paper also presents a simple and feasible method to fabricate continuous helical baffles used for STHXs.

A Numerical Study of Flow and Heat Transfer Enhancement Using an Array of Delta-Winglet Vortex Generators in a Fin-and-Tube Heat Exchanger

Abstract: This work is aimed at assessing the potential of winglet-type vortex generator (VG) "arrays" for multirow inline-tube heat exchangers with an emphasis on providing fundamental understanding of the relation between local flow behavior and heat transfer enhancement mechanisms. Three different winglet configurations in common-flow-up arrangement are analyzed in the seven-row compact fin-and-tube heat exchanger: (a) single-VG pair; (b) a 3VG-inline array (alternating tube row); and (c) a 3VG-staggered array. The numerical study involves three-dimensional time-dependent modeling of unsteady laminar flow (330≤ Re < 850) and conjugate heat transfer in the computational domain, which is set up to model the entire fin length in the air flow direction. It was found that the impingement of winglet redirected flow on the downstream tube is an important heat transfer augmentation mechanism for the common-flow-up arrangement of vortex generators in the inline-tube geometry. At Re=850 with a constant tube-wall temperature, the 3VG-inline-array configuration achieves enhancements up to 32% in total heat flux and 74% in j factor over the baseline case, with an associated pressure-drop increase of about 41%. The numerical results for the integral heat transfer quantities agree well with the available experimental measurements.

Heat Exchanger Efficiency

Abstract: This paper provides the solution to the problem of defining thermal efficiency for heat exchangers based on the second law of thermodynamics. It is shown that corresponding to each actual heat exchanger, there is an ideal heat exchanger that is a balanced counter-flow heat exchanger. The ideal heat exchanger has the same UA, the same arithmetic mean temperature difference, and the same cold to hot fluid inlet temperature ratio. The ideal heat exchanger's heat capacity rates are equal to the minimum heat capacity rate of the actual heat exchanger. The ideal heat exchanger transfers the maximum amount of heat, equal to the product of UA and arithmetic mean temperature difference, and generates the minimum amount of entropy, making it the most efficient and least irreversible heat exchanger. The heat exchanger efficiency is defined as the ratio of the heat transferred in the actual heat exchanger to the heat that would be transferred in the ideal heat exchanger. The concept of heat exchanger efficiency provides a new way for the design and analysis of heat exchangers and heat exchanger networks.

Heat transfer system and method for turbine engine using heat pipes

Abstract: A heat transfer system is provided for a turbine engine of the type including an annular casing (14) with an array of generally radially-extending strut members (20) disposed therein. The heat transfer system includes at least one primary heat pipe (36) disposed at least partially inside a selected one of the strut members (20); at least one secondary heat pipe (48) disposed outside the fan casing (14) and thermally coupled to at least one primary heat pipe (36) and to a heat source. Heat from the heat source can be transferred through the secondary heat pipe (48) to the primary heat pipe (36) and to the selected strut member (20).

Two-phase flow heat transfer of CO2 vaporization in smooth horizontal minichannels

Abstract: Experiments were performed on the convective boiling heat transfer in horizontal minichannels with CO 2 . The test section is made of stainless steel tubes with inner diameters of 1.5 and 3.0 mm and with lengths of 2000 and 3000 mm, respectively, and it is uniformly heated by applying an electric current directly to the tubes. Local heat transfer coefficients were obtained for a heat flux range of 20–40 kW m −2 , a mass flux range of 200–600 kg m −2 s −1 , saturation temperatures of 10, 0, −5, and −10 °C and quality ranges of up to 1.0. Nucleate boiling heat transfer contribution was predominant, especially at low quality region. The reduction of heat transfer coefficient occurred at a lower vapor quality with a rise of heat flux, mass flux and saturation temperature, and with a smaller inner tube diameter. The experimental heat transfer coefficient of CO 2 is about three times higher than that of R-134a. Laminar flow appears in the minichannel flows. A new boiling heat transfer coefficient correlation that is based on the superposition model for CO 2 was developed with 8.41% mean deviation.

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

Abstract: 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.

Effectiveness-ntu computation with a mathematical model for cross-flow heat exchangers

Abstract: - Due to the wide range of design possibilities, simple manufactured, low maintenance and low cost, cross-flow heat exchangers are extensively used in the petroleum, petrochemical, air conditioning, food storage, and others industries. In this paper a mathematical model for cross-flow heat exchangers with complex flow arrangements for determining e-NTU relations is presented. The model is based on the tube element approach, according to which the heat exchanger outlet temperatures are obtained by discretizing the coil along the tube fluid path. In each cross section of the element, tube-side fluid temperature is assumed to be constant because the heat capacity rate ratio C*=C min /C max tends toward zero in the element. Thus temperature is controlled by effectiveness of a local element corresponding to an evaporator or a condenser-type element. The model is validated through comparison with theoretical algebraic relations for single-pass cross-flow arrangements with one or more rows. Very small relative errors are obtained showing the accuracy of the present model. e-NTU curves for several complex circuit arrangements are presented. The model developed represents a useful research tool for theoretical and experimental studies on heat exchangers performance.

Phase-Change Heat Transfer in Microsystems

Abstract: Recent work on miscroscale phase-change heat transfer, including flow boiling and flow condensation in microchannnels (with applications to microchannel heat sinks and microheat exchangers) as well as bubble growth and collapse on microheaters under pulse heating (with applications to micropumps and thermal inkjet printerheads), is reviewed

A New Approach to Numerical Simulation of Small Sized Plate Heat Exchangers With Chevron Plates

Abstract: A numerical and experimental study of heat transfer and fluid flow in a single pass counter flow plate heat exchanger with chevron plates has been presented in this paper. CFD analysis of small sized plate heat exchanger was carried out by taking the complete geometry of the heat transfer surface and more realistic hydrodynamic and thermal boundary conditions. A cold channel with two chevron plates and two halves of hot channels on either side having flat periodic boundaries was selected as the computational domain. The numerical model was validated with data from experiments and empirical correlations from literature. Heat transfer and pressure drop data were obtained experimentally with water as the working fluid, in the Reynolds number range 400-1300 and the Prandtl number range 4.4-6.3.

Heat exchanger: from micro- to multi-scale design optimization

Abstract: This paper presents a consideration of micro-heat exchangers design optimization for the aim of process intensification Two examples are discussed to illustrate different ways of heat transfer intensification in micro-heat exchangers To solve the key issue of the link between the micro-scale and the macro-scale, a multi-scale design optimization method using fractal and constructal approaches is introduced The concept of a novel constructal heat exchanger is also proposed

Micro-channel pulsating heat pipe

Abstract: A heat pipe device and a corresponding method in which micro-channel embedded pulsating heat pipes are incorporated into a substrate. A volume of fluid in a vacuum is introduced into a micro-channel which will become slugs of liquid. Heating of the contents of the micro-channel at an evaporator region (heat source) will cause vaporization within the micro-channel and cooling at a heat sink will cause condensation within the micro-channel, acting to both drive fluid flow within the micro-channel and efficiently transfer heat. Such devices could be used in a number of different configurations, including one as a stacked set of micro-channel embedded substrates.

Mitigation of in-tube fouling in heat exchangers using controlled mechanical vibration

Abstract: Fouling of heat exchange surfaces is mitigated by a process in which a mechanical force is applied to a fixed heat exchanger to excite a vibration in the heat exchange surface and produce shear waves in the fluid adjacent the heat exchange surface. The mechanical force is applied by a dynamic actuator coupled to a controller to produce vibration at a controlled frequency and amplitude output that minimizes adverse effects to the heat exchange structure. The dynamic actuator may be coupled to the heat exchanger in place and operated while the heat exchanger is on line.

Condensation Heat Transfer of R-134A in Horizontal Straight and Helically Coiled Tube-in-Tube Heat Exchangers

Abstract: This article presents an experimental investigation on condensation heat transfer of R-134a in horizontal straight and helically coiled tube-in-tube heat exchangers. The experiments were carried out at three saturation temperatures(35$°, 40$° and 45$°) with the refrigerant mass flux varying from 100 kg/m2 s to 400 kg/m2 s and the vapor quality ranging from 0.1 to 0.8. The effects of vapor quality and mass flux of R-134a on the condensation heat transfer coefficient were investigated. The results indicate that the condensation heat transfer coefficients of the helical section are 4%-13.8% higher than that of the straight section. The experimental results were compared with the data available in literature for helical and straight pipes.

Heat transfer system for turbine engine using heat pipes

Abstract: A heat transfer system is provided for a turbine engine of the type including an annular inlet cowling (12). The heat transfer system includes at least one heat pipe (28) disposed in contact with an interior (10) of the casing. The heat pipe (28) is thermally coupled to a heat source, such that heat from the heat source can be transferred through the heat pipe (28) and into the inlet cowling (12).

Experimental Heat Transfer, pressure drop, and Flow Visualization of R-134a in Vertical Mini/Micro Tubes

Abstract: For the application of minichannel heat exchangers, it is necessary to have accurate design tools for predicting heat transfer and pressure drop. Until recently, this type of heat exchangers was no ...