Showing papers in "Heat Transfer Engineering in 2005"

High Flux Heat Removal with Microchannels—A Roadmap of Challenges and Opportunities

Abstract: Heat fluxes in IC chips and other electronics equipment have reached the current limits of air-cooling technology. Some of the applications require heat fluxes well beyond the limit of 100 W/cm 2 , thus demanding advanced cooling solutions. Liquid cooling technology has also received attention as the advances in single-phase liquid cooling in microchannels have shown considerable promise. The extension of compact heat exchanger technology to microscale applications offers many new possibilities. The liquid cooling technology is expected to reach heat dissipation rates as high as 10 MW/m 2 (1 kW/cm 2 ) with enhanced microchannels and a junction-to-air temperature difference of 50°C. The challenges facing flow boiling systems are also evaluated. This paper reviews the fundamental technological developments in liquid cooling as well as flow boiling and presents possible solutions in integrating the cooling system with a building's HVAC unit in a large server-type application. The opportunities and challenges...

Condensation Pressure Drop in Circular Microchannels

Abstract: This paper presents a multiple flow-regime model for pressure drop during the condensation of refrigerant R134a in horizontal microchannels. Two-phase pressure drops were measured in five circular channels ranging in hydraulic diameter from 0.5 mm to 4.91 mm. For each tube under consideration, pressure drop measurements were first taken over the entire range of qualities from 100% vapor to 100% liquid for five different refrigerant mass fluxes between 150 kg/m2-s and 750 kg/m2-s. Results from the previous work by the authors on condensation flow mechanisms in microchannel geometries were used to assign the applicable flow regime to the data points. Pressure drop models for intermittent [1] and annular [2] flow reported earlier by the authors were modified and combined to develop a comprehensive model that addresses the entire progression of the condensation process from the vapor phase to the liquid phase. This combined model accurately predicts condensation pressure drops in the annular, dispers...

Experimental Study of Flow Patterns, Pressure Drop, and Flow Instabilities in Parallel Rectangular Minichannels

Abstract: Flow boiling heat transfer in parallel minichannels and microchannels is one of the solutions proposed for cooling high heat flux systems. The associated increase in the pressure drop poses a problem that needs to be studied in detail before the small diameter channels can be implemented in practical systems. The pressure drop fluctuations and the flow instability in a network of parallel channels connected by a common header also need to be addressed for the stable operation of flow boiling systems. The current work focuses on studying the flow patterns, pressure drop fluctuations, and flow instabilities in a set of six parallel rectangular minichannels, each with 333μm in hydraulic diameter. Deionized and degassed water was used for all the experiments. The pressure fluctuations are recorded and signal analysis is performed to find the dominant frequencies and their amplitudes. These pressure fluctuations are then mapped to their corresponding flow patterns observed using a high speed camera. T...

Challenges in Cleaning: Recent Developments and Future Prospects

Abstract: There is considerable scope for the optimization of processes subject to fouling by effective management of cleaning. The cleaning cycle starts with the (often complex) material generated during the fouling cycle. The nature of the deposit determines the most appropriate cleaning method, which can often be optimized significantly via the knowledge of the key mechanisms involved in deposit removal. Links between deposit aging and cleanability need to be established and quantified. There is a wide range of cleaning methods available, and attention is focused here on cleaning-in-place (CIP) techniques. Modern instrumentation allows cleaning (and deposit materials behavior) to be probed to greater degree than ever before, but the removal technology is only part of the cleaning process. Monitoring and validation of cleaning are equally important, particularly for the process plant used in flexible manufacture or subject to batch assurance requirements. Individual sensors are unlikely to meet all monit...

Influence of Surface Properties and Characteristics on Fouling in Plate Heat Exchangers

Abstract: The scope of the present work was to investigate the influence of surface properties of corrugated heat exchanger plates on calcium sulpate crystallization fouling from aqueous salt solutions. The applied surface treatment technologies for the heat exchanger plates are ion beam implantation, ion sputtering, carbo-nitriding & oxidizing, and Ni-P-PTFE coating. Ten pairs of Alfa Laval M3 heat exchanger plates were treated with these techniques and used for this study. Other process parameters such as temperature, solution composition, flow velocity, and the corrugation inclination angle of the plates were kept constant in order to allow for a comparison between the different surface treatments. To evaluate the influence of surface roughness on CaSO4 crystallization fouling, one pair of plates was electropolished. Surface energy and surface roughness measurements were carried out to characterize the treated surfaces.

Fouling of Heat Exchangers-New Approaches to Solve an Old Problem

Abstract: For most engineering graduates, the severely fouled equipment shown on the cover page of this issue of Heat Transfer Engineering bears no relation whatsoever to what they have learned in their unde...

An Experimental Study of Flow Condensation Heat Transfer Inside Circular and Rectangular Mini-Channels

Abstract: By using unique experimental techniques and the careful construction of an experimental apparatus, the characteristics of the local heat transfer were investigated using the condensing R134a two-phase flow in horizontal single mini-channels. The circular channels (D h = 0.493, 0.691, and 1.067 mm) and rectangular channels (Aspect Ratio = 1.0; D h = 0.494, 0.658, and 0.972 mm) were tested and compared. Tests were performed for a mass flux of 100, 200, 400, and 600 kg/m2s, a heat flux of 5 to 20 kW/m2, and a saturation temperature of 40°C. In this study, the effect of heat flux, mass flux, vapor qualities, hydraulic diameter, and channel geometry on flow condensation were investigated, and the experimental local condensation heat transfer coefficients are shown. The experimental data of condensation Nusselt number are compared with existing correlations.

Temperature Distribution in Different Materials Due to Short Pulse Laser Irradiation

Abstract: The purpose of this study is to analyze the heat-affected zone in materials such as meat samples, araldite resin-simulating tissue phantoms, and fiber composites irradiated using a mode-locked short pulse laser with a pulse width of 200 ps. The radial surface temperature profiles are compared with that of a continuous wave (CW) laser of the same average power. The short pulse laser results in a more localized heating than a continuous laser with a corresponding high peak temperature. A parametric study addressing the effect of pulse train frequency, material thickness, and amount of scatterers and absorbing agent in the medium and different initial sample temperatures is performed, and the measured temperature profiles are compared with the theoretical non-Fourier hyperbolic formulations and Fourier parabolic heat conduction formulations for both CW and pulsed laser cases.

Effects of Channel Diameter and Liquid Properties on Void Fraction in Adiabatic Two-Phase Flow Through Microchannels

Abstract: Adiabatic two-phase flow experiments have been conducted to investigate the effects of channel diameter and liquid properties on void fraction in horizontal microchannels. Water/nitrogen gas and ethanol–water/nitrogen gas mixtures were pumped through circular microchannels of 50, 75, 100, and 251μm diameter. The ethanol concentration in water was varied to change the surface tension and liquid viscosity. The void fraction data were obtained by an image analysis technique and correlated as a function of homogeneous void fraction. The void fraction data obtained in 50, 75, and 100μm channels conformed well to the correlation in Kawahara et al. [1], but the data for a 251μm diameter channel agreed with the Armand [2] correlation suitable for minichannels. There was no significant effect of liquid properties on void fraction for all channel sizes. These results suggest that the boundary between microchannels and minichannels would lie between 100 and 251μm.

The effect of windbreak walls on the thermal performance of natural draft dry cooling towers

Abstract: A three-dimensional study using the standard k-ϵ turbulence model to simulate airflow in and around a natural draft dry cooling tower (NDDCT) has been conducted using a general-purpose CFD code. This investigation considered the location and the porosity of windbreak walls' structure on the NDDCT thermal performance. In addition, the effect of the windbreak walls on the thermal performance of two NDDCTs with different capacities has been investigated. Two parameters have been used to show the effect of the windbreak walls on the NDDCT thermal performance. At the reference heat exchanger temperature, the thermal effectiveness parameter has been employed. At the reference rejected heat from the NDDCT, the change in the cooling tower approach parameter has been employed. The results in this paper show an improvement in the NDDCT thermal performance due to the introduction of windbreak walls. Moreover, optimizing the location of the windbreak walls has been shown to have a more significant effect on the NDDCT...

Two-Phase Flow Distribution to Tubes of Parallel Flow Air-Cooled Heat Exchangers

Abstract: This paper addresses two-phase flow distribution phenomena in multiple header–tube junctions used in heat exchangers. Because of phase separation, it is very difficult to obtain uniform two-phase flow distribution to the branch tubes. The flow distribution is strongly influenced by the header orientation (horizontal or vertical) and the number of branch tubes. Other factors that influence the flow distribution are the flow direction in the header (upflow or downflow), the header shape and tube end projection into the header, and the location and orientation of the inlet and exit connections. The source of maldistribution is the flow in the dividing headers. Work performed by the authors and others (including patents) are discussed. The possibilities for eliminating two-phase flow maldistribution are identified and discussed. This investigation shows that solutions, which provide uniform flow distribution, are very design-specific. Change of the geometry or operating parameters will require modifi...

Convective Heat Transfer Coefficients in Concentric Annuli

Abstract: The geometric shape of a passage's cross-section has an effect on its convective heat transfer capabilities For concentric annuli, the diameter ratio of the annular space plays an important role The purpose of this study was to determine to what extent research has been done on convective heat transfer in smooth concentric annuli and, if possible, to improve on or contribute to existing theories It was found that although various correlations exist, they are not in good agreement For this study, experiments were conducted with a wide range of annular diameter ratios The Wilson plot method was used to develop a convective heat transfer correlation for annular diameter ratios of 17 to 32 For Reynolds numbers (based on the hydraulic diameter), in the range of 4000 to 30000, the deduced correlation predicted Nusselt numbers accurately within 3% of experimental values

Condensation Heat Transfer and Pressure Gradient Inside Multiport Minichannels

Abstract: In this paper, the experimental heat transfer coefficients measured during condensation of R134a and R410A inside multiport minichannels are presented. The frictional pressure gradient was also measured during adiabatic two-phase flow. The need for experimental research on condensation inside multiport minichannels comes from the wide use of those channels in automotive air-conditioners. The perspective for the adoption of similar channels in the residential air conditioning applications also calls for experimental research on new high pressure refrigerants, such as R410A. Experimental data are compared against models to show the accuracy of the models in the prediction of heat transfer coefficients and pressure drop inside minichannels.

Three-Dimensional Modeling of a Helixchanger® Heat Exchanger Using CFD

Abstract: Detailed three-dimensional computational fluid dynamics (CFD) simulations have been performed to explore the performance of a helically baffled heat exchanger, commercially referred to as the Helixchanger ® heat exchanger. The CFD simulations employ the HEATX computer simulation program, which is designed for the simulation of shell-and-tube heat exchangers. The simulation accounts for the complex helical geometry of baffles, leakages, and nozzle entrance and exit. Three cases are presented that correspond to helix angles of 10°, 25°, and 40°, defined with respect to the radial axis. Inspection of the computed flows reveals distinct inner and outer regions, with the outer region showing a very desirable plug flow characteristic. The inner region, however, displays recirculation zones due to back mixing at the small helix angle, which suggests potential vibration problems but also creates a desirable temperature uniformity. A comparison with plug flow showed that the helically baffled heat exchang...

Numerical Simulation-Aided Parametric Analysis of Natural Convection in a Roof of Triangular Enclosures

Abstract: This work applies an effective and efficient method, called Numerical Simulation-Aided Parametric Analysis (NSAPA), to derive simple and accurate correlation formulae for a natural convective flow system. The classical natural convection in roofs of triangular enclosures is considered to demonstrate the effectiveness of the approach. The buoyancy force generated by the temperature gradient generates a flow pattern. The NSAPA technique derives a set of algebraic equations in characteristic quantities from mass, momentum, and energy conservative equations for natural convection inside a triangular enclosure. The main advantage of the NSAPA technique is that solving the set of algebraic equations is easier than solving the original system of partial differential equations using a numerical method used in computational fluid dynamics. The heat transfer coefficients can be derived using these characteristic quantities. The numerical solution determines the constant factors of the characteristic velocities, the...

Fouling of Some Canadian Crude Oils

Abstract: A thermal fouling study was undertaken using three sour Canadian crude oils Experiments were carried out in a re-circulation fouling loop equipped with an annular (HTRI) electrically heated probe Fluids at pressures of about 1000–1340 kPa under a nitrogen atmosphere were re-circulated at a velocity of 075 m/s for periods of 48 hours, and the decline in heat transfer coefficient followed under conditions of constant heat flux Bulk temperatures were varied over a range of 200–285ˆC, and initial surface temperatures ranged from 300–380ˆC Heat fluxes were in a range of 265–485 kW/m2 Surface temperature effects on fouling of the three oils were compared, and fouling activation energies were estimated For the lightest oil, a more detailed study of velocity and bulk and surface temperature effects was carried out The fouling rate decreased slightly with increasing velocity but increased with both surface and bulk temperatures; a rough correlation was developed using a modified film temperature weighted m

Fouling Characteristics of a Light Australian Crude Oil

Abstract: Australian crude oils, which generally contain little asphaltenes, nevertheless give rise to fouling in refinery pre-heat trains. In this research, the fouling of a series of such crude oils and their blends is being assessed. The present work focuses on thermal fouling resulting from heating Gippsland crude oil at moderate temperatures. The oil is maintained under nitrogen at a pressure of 379 kPa and re-circulated at bulk temperatures of 80–120ˆC through an electrically heated annular probe at velocities in the range 0.25–0.65 m/s with surface temperatures from 180–260ˆC. Experiments are run for periods up to ninety hours at constant heat flux. Fouling is detected by the increase of wall temperature of the probe. The oil is characterized by its filterable solids content, density, and viscosity both before and after the fouling run. The trends in fouling rates are compared to predictions of the threshold-fouling model proposed by Ebert and Panchal [6]. Data on deposit composition are presented, ...

Minichannels in Polymer Electrolyte Membrane Fuel Cells

Abstract: This paper provides an overview of the application of minichannels, typically on the order of a 1 mm hydraulic diameter, in the design of polymer electrolyte membrane (PEM) fuel cells. In these ele...

Refinement of the Transfer Characteristic Correlation of Wet-Cooling Tower Fills

Abstract: Transfer characteristic correlations given in the literature for wet-cooling tower fills are generally only a function of the air and water mass flow rates. This is a gross simplification of a very complex heat and mass transfer (evaporative cooling) process. In addition to the effects of the air and water mass flow rates, effects of the inlet water temperature, air drybulb temperature, wetbulb temperature, and fill height on the transfer characteristic, or Merkel number, are investigated in the present study. The accuracy of two different empirical equations is also evaluated. It is found that the transfer characteristic correlations for wet-cooling tower fills are functions of the inlet water temperature and fill height but not of the air drybulb and wetbulb temperatures.

Fluid Dynamics and Transfer Processes in Bended Microchannels

Abstract: The understanding of the flow processes in microchannels and micromixers is essential for the design of microfluidic devices like microreactors or analytical equipment. We have performed a systematic numerical CFD-study of mixing and mass transfer in sharp 90° bends and heat transfer in T-joints to obtain a detailed insight into the flow patterns and corresponding transfer processes in a wide range of Reynolds numbers. With increasing flow velocity, the straight laminar flow starts to form symmetrical vortices in the bend, at the entrance of the mixing channel, and in T-joints. The vortices enhance the transport processes like heat and mass transfer in the channels significantly. The influence of the geometry and the flow conditions is shown by an analytical estimation of the relevant forces. The appearance of convective transport processes is used for the definition of microflows, which are controlled by viscous forces and diffusive transfer processes.

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.

Numerical Investigation of Convective Heat Transfer in Unsteady Laminar Flow Over a Square Cylinder in a Channel

Abstract: Unsteady laminar flow past a heated square cylinder mounted inside a plane channel was investigated numerically. The blockage ratio was chosen as 1/8 and the Reynolds number based on the mean flow velocity and chord length of the square cylinder was selected as less than 200, for which the two-dimensional behavior of the flow is assured. The time-averaged Nusselt number as well as some integral parameters such as drag coefficient, recirculation length, and Strouhal number were obtained and compared with literature. Results show a nearly linear increase in recirculation length and decrease in drag coefficient with increasing Reynolds number for the steady flow regime. There is an increase in the total Nusselt number and drag coefficient with a Reynolds number for unsteady flow regime, where vortex shedding is observed from the cylinder. A correlation was obtained for the variation of the total Nusselt number with the Reynolds number.

Fouling During the Use of Seawater as Coolant—the Development of a User Guide

Abstract: ESDU International of London, UK h as recently published a 'User Guide' (1) on fouling in heat exchange systems using seawater as the coolant. Developed over a period of eighteen months in close collaboration with a team of oil refiners, heat transfer equipment and services suppliers and u niversities, with valuable input from the power industry, the User Guide is a practical guide to the current state of knowledge relating to fouling in cooling systems using seawater. Its objective is to provide the designer and the operator of both onshore and offshore equipment with a practical source of guidance on the occurrence, the mechanisms and the mitigation of seawater fouling in these systems. ESDU's collaborative Oil Industry Fouling Working Party was formed in recognition of the huge economic a nd environmental importance of heat exchanger fouling and the potential benefits that can accrue from better understanding of mitigation strategies. The seawater fouling User Guide is the second in a group, following the development of the Crude Oil Foulin g User Guide issued in 2000. Work is now underway on cooling water fouling, The development of the User Guide is discussed in this paper and its technical content is summarized

Experimental study of heat transfer and pressure drop characteristics of air/water and air-steam/water heat exchange in a polymer compact heat exchanger

Abstract: Experiments of heat transfer and pressure drop in a polymer compact heat exchanger made of PolyVinyliDene-Fluoride were conducted under various conditions for air/water heat exchange and air-steam/water heat exchange, respectively. The overall heat transfer coefficients of air-steam/water heat exchange (150 to 600 W/m2K) are much greater than that of air/water heat exchange (80 to 130 W/m2K). The pressure drops of air-steam are also greater than that of dry air in the test range. The gas-side heat transfer coefficients were obtained with the Wilson plot technique. The heat transfer coefficients of dry air range from 80 to 150 W/m2K, which are in the same magnitude grade as the overall heat transfer coefficients. It shows that the major heat resistance is in the gas side. The heat transfer coefficients of air-steam range from 400 to 24000 W/m2K at various inlet conditions. The inlet temperatures and humidities have a great effect on the heat transfer coefficients. In reality, it is the inlet steam...

Retrofitting Crude Oil Refinery Heat Exchanger Networks to Minimize Fouling While Maximizing Heat Recovery

Abstract: The use of fouling factors in heat exchanger design and the lack of appreciation of fouling in traditional pinch approaches have often resulted in crude preheat networks that are subject to extensive fouling The development of thermal and pressure drop models for crude oil fouling has allowed its effects to be quantified so that techno-economic analyses can be performed and design options compared The application of these fouling models is described here on two levels: the assessment of increasing heat recovery in stream matches (eg, by adding extra area to exchangers) and the design of a complete network using the Modified Temperature Field Plot Application to a refinery case study showed that, at both the exchanger and network levels, designing for maximum heat recovery (eg, using traditional pinch approaches) results in a less efficient system over time due to fouling effects

Biocide Dosing Strategies for Biofilm Control

Abstract: In order to reduce the environmental impact of biocide for the control of biofilm formation in cooling water circuits, “environmentally friendly” biocides have been developed; however, they are generally more expensive than traditional chemicals. It is imperative, therefore, that the minimum quantity of biocide is employed so that costs are kept to a minimum. To achieve this objective, optimum dosing strategies are required. Using a pilot plant in conjunction with a monoculture of Pseudomonas fluorecsens as the biofouling bacterium, tests were carried out using a proprietary biocide to investigate the effects of dose concentration, duration, and frequency of dosing and fluid mechanics on biofilm control. With four fifteen-minute applications per day at a peak concentration of 16.8 mg/l, it was not possible to inhibit biofilm development. Control was effected, however, by doubling the peak concentration using a short dosing period. As would be expected, concentration was shown to be a critical fac...

The Role of Heat Transfer in Refrigeration

Abstract: Refrigeration plays a key role in today's society, providing human comfort and food preservation. From ice and large food-packing plants to oil refineries and chemical manufacturing, industrial pro...

A review of concentric annular heat pipes

Abstract: A detailed description of a concentric annular heat pipe (CAHP) operation is presented in low to moderate temperature ranges (50–200°C). The steady-state response of a CAHP to various heat fluxes in the evaporator and condenser sections are discussed. Two-dimensional mathematical modeling of the fluid flow and heat transfer in the annular vapor space and the wicks are described. The fundamental aspects and limitations of the operation of a CAHP are also discussed. Previously used numerical and experimental approaches for the analysis of the CAHPs and some related concepts are reviewed. The Navier-Stokes and similar equations are recommended for the simulation of fluid flow and heat transfer in the annular vapor space and the wicks. A number of important concepts, such as two-phase flow, heat transfer in the heat pipe wicks, the vapor–liquid interface conditions, design considerations, testing, and manufacture of a CAHP, are also briefly discussed. The results of this research have shown that the ...

Heat Transfer Characteristics during Mist Cooling on a Heated Cylinder

Abstract: Mist cooling is expected to contribute to better steel products because it enables uniform and moderate cooling in steel-making processes. In this study, experimental data were obtained to understand the mist cooling process in a high-temperature cylinder. The wetting phenomenon on the cylinder surface was also observed with an installed CCD camera. Comparing these results with those from previous investigations, we found that our cooling curve corresponded to two distinct heat transfer regimes in mist cooling. A simplified model from the analysis was introduced to simulate the cooling curve in each heat transfer regime. The estimated cooling curves by this model agreed with the experimental data in one regime and had a similarity in the other regime.

Heat transfer during annular tube contact in a helically coiled tube-in-tube heat exchanger

Abstract: Helically wound tube-in-tube heat exchangers are manufactured by coiling two tubes, one placed inside the other. This method often results in the tubes not sharing the same center line, and therefore annular contact occurs in some cases. An experimental comparison was made of such tubes in a heat exchanger with annular contact, as opposed to an aligned (concentric) device without annular contact, in order to quantify the effect of annular contact in terms of heat transfer coefficients and pressure drop. By comparing the heat transfer characteristics, it was concluded that the heat transfer coefficient in the annulus was found to increase substantially. The result was an improved performance by the heat exchanger where annular contact occurs, compared to the heat exchanger with the inner tube in a concentric position.