Showing papers in "Heat Transfer Engineering in 2011"

Heat Exchanger Fouling: Mitigation and Cleaning Strategies

Abstract: Heat exchangers are the workhorse of most chemical, petrochemical, food-processing, and power-generating processes. The global heat exchanger market is estimated to top a total of $12.7 billion by ...

A Review of Cooling in Microchannels

Abstract: Advancements in electronic performance result in a decrease in device size and increase in power density. Because of these advancements, current cooling mechanisms for electronic devices are beginning to be ineffective. Within the localized hot spots, the materials of the components are reaching temperature values that can lead to improper functioning of the device. Many techniques have been successful in the past, such as heat sinks, cavities or grooves, micro pin-fins, etc., but still do not provide adequate cooling necessary to maintain temperature values low enough for the electronic components to operate. Microchannels, with their large heat transfer surface to volume ratio, cooled with either gas or liquid coolant, have shown some potential. By modifying the walls of the microchannel with fins, pins, or grooves, the cooling performance can be improved. A possible fin material used to increase the surface area of a microchannel is carbon nanotubes, which possess excellent thermal and mechanical prope...

Nanoscale Surface Modification Techniques for Pool Boiling Enhancement—A Critical Review and Future Directions

Abstract: New discoveries presented in the last decade for enhancing boiling performance utilizing nanoscale structures on surfaces are critically examined in this paper. Since the mechanism for such a phenomenon is not fully understood, this review mainly focuses on the experimental studies reported in the literature on the boiling phenomena on nanostructures, and implementation of nanostructures on various substrates. The paper also focuses on the interpretation of underlying phenomena for enhancing the boiling performance. The main influencing parameter in controlling is seen as the change in the surface energy of the boiling surface, which is characterized by the contact angle of the liquid and vapor phase interface at the heating surface. The nanostructures are seen to alter the contact angle. Design consideration and theoretical developments are also discussed, followed by practical aspects of nanostructure manufacturing. The issues related to performance, ease of fabrication, and durability (whenever availab...

The Bayesian Framework for Inverse Problems in Heat Transfer

Abstract: The aim of this paper is to provide researchers dealing with inverse heat transfer problems a review of the Bayesian approach to inverse problems, the related modeling issues, and the methods that are used to carry out inference. In Bayesian inversion, the aim is not only to obtain a single point estimate for the unknown, but rather to characterize uncertainties in estimates, or predictions. Before any measurements are available, we have some uncertainty in the unknown. After carrying out measurements, the uncertainty has been reduced, and the task is to quantify this uncertainty, and in addition to give plausible suggestions for answers to questions of interest. The focus of this review is on the modeling-related topics in inverse problems in general, and the methods that are used to compute answers to questions. In particular, we build a scene of how to handle and model the unavoidable uncertainties that arise with real physical measurements. In addition to giving a brief review of existing Bayesian tre...

Heat Transfer Fouling: 50 Years After the Kern and Seaton Model

Abstract: Fouling of heat exchangers is a chronic problem in processing industries. In addition to the appropriate selection of operating conditions and exchanger geometry, there are numerous chemical and mechanical methods to mitigate fouling and to remove deposits from the heat transfer surfaces. However, all methods to reduce fouling require some understanding of the mechanisms of the deposition process and of the structure and adhesion of deposits on the heat transfer surfaces. Almost exactly 50 years ago, D. Q. Kern and his co-author, R. E. Seaton, published a paper attempting to describe the growth of fouling deposits in terms of an unsteady-state heat and mass balance for the heat transfer surface. More or less at the same time, the TEMA fouling resistances were published based on operational and anecdotal evidence of fouling for a range of heat exchanger applications. These two approaches have since formed the basis for most heat transfer fouling models and heat exchanger designs. Increased costs of energy,...

Comparison of Void Fraction Correlations for Different Flow Patterns in Upward Vertical Two-Phase Flow

Abstract: A comparison of the performance of 52 void fraction correlations was made based on an unbiased experimental data set of 1208 data points. A comprehensive literature search was undertaken for the available void fraction correlations and experimental void fraction data for upward vertical two-phase flow. The performance of the correlations in correctly predicting the diverse data set was evaluated. Comparisons between the correlations were made and appropriate recommendations were drawn. The analysis showed that most of the correlations developed are very restricted in terms of handling a wide variety of data sets. Based on this analysis, void fraction correlations with the best predictive capability are highlighted.

Fouling in Crude Oil Preheat Trains: A Systematic Solution to an Old Problem

Abstract: A major cause of refinery energy inefficiency is fouling in preheat trains. This has been a most challenging problem for decades, due to limited fundamental understanding of its causes, deposition mechanisms, deposit composition, and impacts on design/operations. Current heat exchanger design methodologies mostly just allow for fouling, rather than fundamentally preventing it. To address this problem in a systematic way, a large-scale interdisciplinary research project, CROF (crude oil fouling), brought together leading experts from the University of Bath, University of Cambridge, and Imperial College London and, through IHS ESDU, industry. The research, coordinated in eight subprojects blending theory, experiments, and modeling work, tackles fouling issues across all scales, from molecular to the process unit to the overall heat exchanger network, in an integrated way. To make the outcomes of the project relevant and transferable to industry, the research team is working closely with experts from many world leading oil companies. The systematic approach of the CROF project is presented. Individual subprojects are outlined, together with how they work together. Initial results are presented, indicating that a quantum progress can be achieved from such a fundamental, integrated approach. Some preliminary indications with respect to impact on industrial practice are discussed.

An Inverse Analysis for Parameter Estimation Applied to a Non-Fourier Conduction–Radiation Problem

Abstract: Retrieval of parameters in a non-Fourier conduction and radiation heat transfer problem is reported. The direct problem is formulated using the lattice Boltzmann method (LBM) and the finite-volume method (FVM). The divergence of radiative heat flux is computed using the FVM, and the LBM formulation is employed to obtain the temperature field. In the inverse method, this temperature field is taken as exact. Simultaneous estimation of parameters, namely, the extinction coefficient and the conduction–radiation parameter, is done by minimizing the objective function. The genetic algorithm (GA) is used for this purpose. The accuracies of the estimated parameters are studied for the effects of measurement errors and genetic parameters such as the crossover and mutation probabilities, the population size, and the number of generations. The LBM-FVM in combination with GA has been found to provide a correct estimate of parameters.

Numerical Simulation of Forced Convection Enhancement in a Pipe by Porous Inserts

Abstract: The effect of porous inserts on forced convection in a circular pipe is investigated numerically. Two configurations are considered: A porous material is inserted at the core of the pipe, and an annulus porous material is attached to the inner wall. The flow inside the porous material is modeled using the Darcy–Brinkman–Forchheimer model. Effects of porous thickness, Darcy number, and thermal conductivity on the Nusselt number are investigated. In the first configuration, increasing porous thickness increases Nusselt number, and the value of porous thickness that maximizes Nusselt number varies from 0.8 to 0.95 as the value of Darcy number decreases from 10−3 to 10−6. In the second configuration, for low values of thermal conductivity, increasing the porous thickness decreases Nusselt number, and the porous thickness that achieves the lowest Nusselt number varies from 0.6 to 0.85 as the value of Darcy number decreases from 10−3 to 10−6. However, for high values of thermal conductivity, increasing porous t...

Fouling Problems in Exhaust Gas Recirculation Coolers in the Automotive Industry

Abstract: Transportation is responsible for approximately 20% of global greenhouse gas emissions, such as CO2, NOx, and hydrocarbons that have not been burned completely in the engine. In particular, 55% of globally emitted NOx, which is more harmful to the environment than CO2, is produced by the automotive industry alone. Strict emission standards are now in place that set specific limits to the amount of pollutants that can be released into the environment. The widely used measure to reduce NOx emissions in diesel engines is to return part of the exhaust gas to the intake of the engine. This is usually done through a heat exchanger known as an exhaust gas recirculation (EGR) cooler. However, EGR coolers are subject to severe fouling such that their thermal efficiency can drop by as much as 30% within a very short period of time. More importantly, the deposit layer is a blend of particulate matter and sticky heavy hydrocarbons that are very difficult to remove from the heat exchanger surfaces. The present study a...

Numerical Investigation of Liquid Water Cooling for a Proton Exchange Membrane Fuel Cell Stack

Abstract: The operation of proton exchange membrane fuel cell (PEMFC) stacks requires careful thermal and water management for optimal performance. Appropriate placement of cooling plates and appropriate cooling conditions are therefore essential. To study the impact of these design parameters, a two-phase model accounting for the conservation of mass, momentum, species, energy, and charge, a phenomenological model for the membrane, and an agglomerate model for the catalyst layer, is developed and solved. The model is validated for a single cell, in terms of both the local and the global current density, and good agreement is found. Four repetitive computational units are then identified for the number of single cells placed between the coolant plates: (i) one cell; (ii) two cells; (iii) three cells; and (iv) four cells. The flow fields in the single cells and the cooling plates are of a net type. The results show that there is a strong correlation between stack performance and the operating conditions/placement of the coolant plates. For the limiting case of one coolant plate between each unit cell, similar operating conditions can be achieved in every individual cell throughout the stack. As more cells are placed in between coolant plates, the stack performance drops due to an increase in temperature and decrease in water content in the membranes, unless the cooling temperature is lowered. The coolant temperature and inlet velocity need to be monitored carefully and adjusted to the operating conditions of the stack. This model can be employed for design and optimization of liquid water cooling of a PEMFC stack.

Inverse Heat Transfer Problems

Abstract: Inverse heat transfer problems deal with the estimation of unknown quantities appearing in the mathematical formulation of physical processes in thermal sciences, by using measurements of temperatu...

Thermal Radiative Properties of Complex Media: Theoretical Prediction Versus Experimental Identification

Abstract: In many engineering applications and natural phenomena, thermal radiation interacts with complex media composed of dispersed phases that may be of different type: solid/solid, solid/gas, or liquid/gas. Most of them are semitransparent media that emit, absorb, and scatter thermal radiation. Heat transfer by combined radiation with conduction or convection in such media is a problem of high practical importance, mostly in situations where radiation is a dominant mode. Improvement of thermal performance of such materials or of the manufacturing processes that involve these media requires the availability of efficient methods (i) for radiative transfer modeling, and (ii) to predict and to experimentally determine the thermophysical properties intended to feed the models. This paper is focused on radiative properties assessment. After a brief overview of the materials and properties of interest, the emphasis is put on methodology of property investigation combining both theoretical prediction and experimental identification. Examples related to different particulate media are presented, showing recent advances and needs for further investigation.

Experimental Studies on Heat Transfer and Friction Factor Characteristics of Al2O3/Water Nanofluid in a Circular Pipe Under Transition Flow With Wire Coil Inserts

Abstract: Heat transfer and friction factor characteristics in a circular tube fitted with wire coil inserts are investigated experimentally using Al2O3/water nanofluid as the working fluid. The effects of the pitch ratio and nanofluid on the Nusselt number and the friction factor are determined in a circular tube with fully developed transition flow for Reynolds number in the range of 2500 to 5000. The experiments were performed using wire coil inserts having different pitch ratios with Al2O3/water nanofluid having 0.1% volume concentration of nanoparticles as the working fluid. Experiments using the plain tube and with wire coil inserts were also carried out with distilled water as the working fluid for experimental setup validation and comparison. The experimental results reveal that the use of nanofluids increases the heat transfer rate with negligible increase in friction factor in the plain tube and the tube fitted with wire coil inserts. In addition, empirical correlations are proposed based on the experimen...

A Tool for Predicting the Thermal Performance of a Diesel Engine

Abstract: This paper presents a thermal network model for the simulation of the transient response of diesel engines. The model was adjusted by using experimental data from a completely instrumented engine run under steady-state and transient conditions. Comparisons between measured and predicted material temperatures over a wide range of engine running conditions show a mean error of 7°C. The model was then used to predict the thermal behavior of a different engine. Model results were checked against oil and coolant temperatures measured during engine warm-up at constant speed and load, and on a New European Driving Cycle. Results show that the model predicts these temperatures with a maximum error of 3°C.

Experimental Analysis of the Effects of Particulate Fouling on Heat Exchanger Heat Transfer and Air-Side Pressure Drop for a Hybrid Dry Cooler

Abstract: It is well known that significant fouling by particulate matter can have a deleterious effect on the performance of enhanced surface heat exchangers, and the same is true for hybrid heat exchangers. Hybrid heat exchangers are heat exchangers that are typically run in dry mode to reject heat. When the ambient conditions require more heat rejection than can be provided by sensible heat transfer, a water pump is turned on and water flows over the fins, and the evaporation of water provides a further cooling effect. Fouling in dry-mode operation is physically similar to that of air-cooled heat exchangers, but in evaporative mode the flow of the water over the coil eliminates the impact of fouling. A hybrid dry cooler heat exchanger of 60 cm × 60 cm frontal area has been installed in a well-instrumented wind tunnel to measure the heat exchanger's performance. Hot water flows through the coil to provide the load, and air flows over the coil to provide cooling. During evaporative mode operation another stream of...

A Mathematical Model of an Oscillating Heat Pipe

Abstract: A mathematical model predicting the oscillating motion in an oscillating heat pipe is developed. The model considers the system multidegree oscillation of vapor bubbles and liquid plugs, including the effects of filling ratio, operating temperature, gravitational force, and temperature difference between the evaporator and condenser. The model shows that the average velocity of liquid slugs is determined by the temperature difference between the evaporator and condenser. As the turn number increases, the temperature difference for the system to start the oscillating motion decreases. Increasing the bubble number will make the system more unstable and the system can be easily started up. The existence of gravity at the bottom heating mode will make the system easily produce the oscillating motion and decrease the temperature difference as well. Results presented here will assist in optimizing the heat transfer performance and provide a better understanding of heat transfer mechanisms occurring in the oscil...

Condensation Heat Transfer and Pressure Losses of High- and Low-Pressure Refrigerants Flowing in a Single Circular Minichannel

Abstract: A 0.96 mm circular minichannel is used to measure both heat transfer coefficients during condensation and two-phase pressure losses of the refrigerants R32 and R245fa. Test runs have been performed at around 40°C saturation temperature, corresponding to 24.8 bar saturation pressure for R32 and 2.5 bar saturation pressure for R245fa. The pressure drop tests have been performed in adiabatic flow conditions, to measure only the pressure losses due to friction. The heat transfer experimental data are compared against predicting models to provide a guideline for the design of minichannel condensers.

Characterization of Crude Oils and Their Fouling Deposits Using a Batch Stirred Cell System

Abstract: A small (1 L) batch stirred cell system has been developed to study crude oil fouling at surface temperatures up to 400°C and pressures up to 30 bar. Fouling resistance–time data are obtained from experiments in which the principal operating variables are surface shear stress, surface temperature, heat flux, and crude oil type. The oils and deposits are characterized and correlated with the experimental heat transfer fouling data to understand better the effects of process conditions such as surface temperature and surface shear stress on the fouling process. Deposits are subjected to a range of qualitative and quantitative analyses in order to gain a better insight into the crude oil fouling phenomenon. Thermal data that can be obtained relatively quickly from the batch cell provide fouling rates, Arrhenius plots, and apparent activation energies as a function of process variables. The experimental system, supported by computational fluid dynamics (CFD) studies, allows fouling threshold conditions of sur...

A common method for calculation of flow boiling and flow condensation heat transfer coefficients in minichannels with account of nonadiabatic effects

Abstract: Flow boiling and flow condensation are often regarded as two opposite or symmetrical phenomena; however, their description with a single correlation has yet to be suggested. In the case of flow boiling in minichannels there is mostly encountered the annular flow structure, where bubble generation is not present. A similar picture holds for the case of inside tube condensation, where annular flow structure predominates. In such a case the heat transfer coefficient is primarily dependent on the convective mechanism. In this article a method developed earlier by the authors is applied to calculations of the heat transfer coefficient for flow condensation. The modifications of interface shear stresses between flow boiling and flow condensation are considered through incorporation of the so-called blowing parameter, which differentiates between these two modes of heat transfer. Satisfactory consistency with well-established correlations for condensation has been found, as well as with selected experimental data.

Two-Phase Frictional Pressure Drop and Flow Boiling Heat Transfer for R245fa in a 2.32-mm Tube

Abstract: Experimental two-phase frictional pressure drop and flow boiling heat transfer results are presented for a horizontal 2.32-mm ID stainless-steel tube using R245fa as working fluid. The frictional pressure drop data was obtained under adiabatic and diabatic conditions. Experiments were performed for mass velocities ranging from 100 to 700 kg m−2 s−1, heat flux from 0 to 55 kW m−2, exit saturation temperatures of 31 and 41°C, and vapor qualities from 0.10 to 0.99. Pressures drop gradients and heat transfer coefficients ranging from 1 to 70 kPa m−1 and from 1 to 7 kW m−2 K−1 were measured. It was found that the heat transfer coefficient is a strong function of the heat flux, mass velocity, and vapor quality. Five frictional pressure drop predictive methods were compared against the experimental database. The Cioncolini et al. (2009) method was found to work the best. Six flow boiling heat transfer predictive methods were also compared against the present database. Liu and Winterton (1991), Zhang et al. (2004...

Transitional Flow and Related Transport Phenomena in Curved Microchannels

Abstract: In microchannels with typical dimensions from 10 μm to a few hundreds of micrometers, the flow is dominated by viscous forces, often leading to laminar flow conditions. At the entrance or in bends and curves, where the flow changes its velocity or direction, inertial forces generate transverse flow velocities. Due to continuity, vortex pairs, such as Dean flow in circular bends, are generated, which are still laminar, steady, and showing no statistically distributed fluctuations typical for turbulent flow. This deviation from straight laminar conditions is called transitional flow, often occurring in channels larger than 500 μm at higher flow rates. Transitional flow phenomena include the first occurrence of flow bifurcation, pulsating vortices, period doubling of vortex pairs, and regularly fluctuating wake flow or vortex shedding. Chaotic flow phenomena are the first evidence of turbulence. Transitional flow augments the transport characteristics in microchannels for enhanced heat and mass transfer and ...

Boiling of R404A Refrigeration Medium Under the Conditions of Periodically Generated Disturbances

Abstract: An attempt was made to evaluate the impact of external, periodically generated disturbances on the boiling process of the refrigeration medium in a flow. The experimental investigations were conducted under the conditions of periodic changes (increase and fading) of the mass flux density of the refrigeration medium for constant refrigeration chamber heat loads. This led to a change of the pressure and temperature along the path of the flow of the medium in a coil tube of the evaporator. It was confirmed that the boiling process of the refrigeration medium in a flow exhibits wave properties, which are characterized by finite values of the displacements of the disturbances. By way of dimensional analysis, nondimensional dependences were determined that specify the velocity of the displacement of the pressure change signal and the temperature change signal. The investigations were conducted with the use of an environmentally friendly R404A refrigeration medium.

Thermal Performance Assessment of Turbulent Tube Flow Through Wire Coil Turbulators

Abstract: This paper presents characteristics of turbulent convective heat transfer in a tube fitted with wire coil turbulators. Two different wire coils are introduced: (1) with typical/uniform coil pitch ratio (CR) and (2) with periodically varying three-coil pitch ratio. Various uniform coil pitch ratios (CR = 4, 6, and 8) and two periodically varying coil pitch ratios, the D-coil (decreasing three-coil pitch ratio arrangement) and DI-coil (decreasing/increasing three-coil pitch ratio arrangement), are experimentally investigated in a uniform heat flux tube. The experiments are performed for turbulent flows with Reynolds numbers ranging between 4500 and 20,000. All of the experimental results are compared with those obtained from using the plain tube, while the thermal performance factor is evaluated under an equal pumping power constraint. The experimental results show that the use of the tube fitted with all wire coils leads to an advantage on the basis of heat transfer enhancement over the plain tube with no ...

Three-Dimensional Molecular Dynamic Study on Accommodation Coefficients in Rough Nanochannels

Abstract: The tangential momentum and energy accommodation coefficients (TMAC and EAC) are parameters to characterize the velocity slip and temperature jump in the gas–solid interface. To understand the wall effects on fluid flow and heat transfer in nanochannels, the accommodation coefficients for argon gas molecules and platinum wall atoms were calculated according to a proper statistical algorithm using a three-dimensional molecular dynamic method. Isothermal flows and thermal conductions were simulated in smooth and rough nanochannels, in which the roughness ranged from 0.2 nm to 1.4 nm. From the atomic viewpoint, different lattice arrangements of smooth walls would induce atomic roughness to different extents on the surface, which affected the momentum and energy exchange in gas–wall interactions and resulted in different accommodation coefficients. In channels with nanoscale roughness, the possibility of multiple gas–wall interactions were further increased so that the TMAC and EAC became much larger with mor...

Flow Boiling of R245fa in a Single Circular Microchannel

Abstract: This paper describes an experimental setup for the investigation of two-phase heat transfer inside microchannels and reports local heat transfer coefficients measured during flow boiling of HFC-245fa in a 0.96-mm-diameter single circular channel. The test runs have been performed during vaporization at around 1.85 bar, corresponding to 31°C saturation temperature. As a peculiar characteristic of the present technique, the heat transfer coefficient is not measured by imposing the heat flux; instead, the boiling process is governed by controlling the inlet temperature of the heating secondary fluid. In the data, mass velocity ranges between 200 and 400 kg m−2 s−1, with heat flux varying from 5 to 85 kW m−2 and vapor quality from 0.05 up to 0.8. Since these data are not measured at uniform heat flux conditions, a proper analysis is performed to enlighten the influence of the different parameters and to compare the present data to those obtained when the heat flux is imposed. Besides, the test runs have been ...

Approach for the Determination of Heat Transfer Coefficients for Filling Processes of Pressure Vessels With Compressed Gaseous Media

Abstract: For fast and effective simulation of filling processes of pressure vessels with compressed gaseous media, the governing equations are derived from a mass balance equation for the gas and from energy balance equations for the gas and the wall of the vessel. The gas is considered as a perfectly mixed phase and two heat transfer coefficients are introduced. The first one is the mean heat transfer coefficient between the gas and the inner surface of the pressure vessel, and the second one is the heat transfer coefficient between outer surface of the vessel and the surroundings. Because of the heat capacity of the wall of the pressure vessel, heat transfer from the compressed gas to the vessel wall strongly influences the temperature field of the gas. Until now no correlations have been available for the heat transfer coefficient between inflowing gas and inner surface of the vessel. To solve this problem, a computational fluid dynamics tool is used to determine the gas velocities at the vicinity of the inner ...

Effect of Inlet Conditions on Taylor Bubble Length in Microchannels

Abstract: The effect of inlet conditions on the frequency and size of the bubbles that form during gas–liquid Taylor flow in microchannels is investigated in this paper. Three different inlet configurations, T-, Y-, and M- junction, and three test channels with hydraulic diameters 0.345 mm, 0.577 mm, and 0.816 mm were used. The test fluids were nitrogen and water or octane, which have different surface tensions. It was found that bubble length increased with increasing gas flow rate, gas inlet size, and liquid surface tension and with decreasing liquid flow rate. From the different inlet configurations, the M-junction resulted in the largest bubbles and the Y-junction in the smallest ones, particularly at low liquid flow rates. Bubble lengths calculated from experimental bubble formation frequencies were tested against a number of literature correlations but the agreement was not very good. Two new correlations were developed for the T- and the Y-junctions to calculate the unit cell (one bubble and one slug) freque...

Reconstruction of the Space- and Time-Dependent Blood Perfusion Coefficient in Bio-Heat Transfer

Abstract: The identification of the space- and time-dependent perfusion coefficient in the one-dimensional transient bio-heat conduction equation is investigated. While boundary and initial conditions are prescribed, additional temperature measurements are considered inside the solution domain. The problem is approached both from a global and a local perspective. In the global approach a Crank–Nicolson-type scheme is combined with the Tikhonov regularization method. In the local approach, we compute both the time first-order and space second-order derivatives by means of first kind integral equations. A comparison between the numerical results obtained using the two methods shows that the local approach is more accurate and stable than the global one.

Refinery Pre-Heat Train Network Simulation Undergoing Fouling: Assessment of Energy Efficiency and Carbon Emissions

Abstract: Fouling in crude preheat trains in oil refineries causes additional fuel and production costs, operating difficulties, CO2 emissions, and safety issues. Crude oil fouling deposition mechanisms are still not well understood. Current exchanger design methodologies (based on empirical fouling factors), operating practices, and mitigation solutions (ranging from the use of chemical additives to tube inserts) do not prevent efficiency losses or disruption of operations. Moreover, current analysis and design methodologies neglect local effects and dynamics of fouling, in favor of lumped, steady-state, “averaged” heuristic models. In this paper, a dynamic and distributed model recently developed that accounts for localized fouling growth as a function of process conditions is used to simulate the dynamic behavior of the hot end of a refinery preheat train. The network is simulated by a simultaneous solution of all exchangers, combined according to a desired configuration, within gPROMS, a commercial dynamic simu...