Showing papers in "Heat Transfer Engineering in 2009"
TL;DR: In this article, an experimental rig is developed to investigate and evaluate PV module performance with the proposed cooling technique, and the experimental results indicated that due to the heat loss by convection between water and the PV panel's upper surface, an increase of about 15% in system output is achieved at peak radiation conditions.
Abstract: An effective way of improving efficiency and reducing the rate of thermal degradation of a photovoltaic (PV) module is by reducing the operating temperature of its surface. This can be achieved by cooling the module and reducing the heat stored inside the PV cells during operation. In this paper, long-term performance modeling of a proposed solar-water pumping system is carried out. The system, which is used for irrigation purposes, consists of a PV module cooled by water, a submersible water pump, and a water storage tank. Cooling of the PV panel is achieved by introducing water trickling configuration on the upper surface of the panel. An experimental rig is developed to investigate and evaluate PV module performance with the proposed cooling technique. The experimental results indicated that due to the heat loss by convection between water and the PV panel's upper surface, an increase of about 15% in system output is achieved at peak radiation conditions. Long-term performance of the system is estimate...
253 citations
TL;DR: In this article, the authors provide an overview of the various possible mechanisms reported in the literature that contribute to enhanced thermal conductivity of nanofluids and discuss the existing controversies in them.
Abstract: Due to superior thermal properties and many potential applications, nanofluids have recently attracted the attention of the research community. Though the experiments in nanofluids have revealed the spectacular heat transfer enhancement in them, there are many inconsistencies in the reported experimental values and controversies in the mechanisms proposed for heat transport in nanofluids. This indicates that it is crucial to be aware of the different mechanisms contributing to superior thermal properties of nanofluids and discuss the existing controversies in them. Therefore, in this article, we provide an overview of the various possible mechanisms reported in the literature that contribute to enhanced thermal conductivity of nanofluids. The other factors affecting the nanofluids thermal conductivity and the mathematical models for estimating nanofluids thermal conductivity with their limitations are also discussed.
187 citations
TL;DR: The flow regimes of symmetrical 1:1 mixing in T-shaped micromixers with rectangular cross-sections for Reynolds numbers (Re) from 0.01 to 1000 in the mixing channel are discussed in this article.
Abstract: Convective micromixers create vortices in curved channel elements allowing characteristic mixing times below 1 millisecond for gaseous and liquid media. This contribution gives an overview about the flow regimes of symmetrical 1:1 mixing in T-shaped micromixers with rectangular cross-sections for Reynolds numbers (Re) from 0.01 to 1000 in the mixing channel. First, symmetrical vortices are formed for Re > 10 in the so-called Dean flow. At Re > 140, fluid from one side swaps to the opposite side and creates a double vortex, which enhances mixing. For Re > 240, the flow becomes unsteady, and a kind of wake flow establishes. From 240 < Re < 500, the wake flow is periodic with a Strouhal number of about 0.23. The mixing quality also shows a periodic behavior and reaches its maximum at this point. With further increasing the Re number, the flow becomes chaotic, and the two components are often flowing parallel in the mixing channel, which decreases the mixing quality. Besides detailed CFD simulations, the peri...
147 citations
TL;DR: In this article, a proven correlation for heat transfer in circular tubes was extended by factors that take into consideration the effect of the diameter ratio of the annulus and the different boundary conditions for heating or cooling.
Abstract: On the basis of a large number of experimental data from the literature, correlations were developed for the heat transfer coefficient for turbulent flow in concentric annular ducts. A proven correlation for heat transfer in circular tubes was extended by factors that take into consideration the effect of the diameter ratio of the annulus and the different boundary conditions for heating or cooling.
129 citations
TL;DR: In this article, a detailed analysis of microchannel/microgap heat transfer data for two-phase flow of refrigerants and dielectric liquids, gathered from the open literature and sorted by the Taitel and Dukler flow regime mapping methodology, is performed.
Abstract: A detailed analysis of microchannel/microgap heat transfer data for two-phase flow of refrigerants and dielectric liquids, gathered from the open literature and sorted by the Taitel and Dukler flow regime mapping methodology, is performed. Annular flow is found to be the dominant regime for this thermal transport configuration and to grow in importance with decreasing channel diameter. A characteristic M-shaped heat transfer coefficient variation with quality (or superficial velocity) for the flow of refrigerants and dielectric liquids in miniature channels is identified. The inflection points in this M-shaped curve are seen to equate approximately with flow regime transitions, including a first maximum at the transition from Bubble to Intermittent flow and a second maximum at moderate qualities in Annular flow, just before local dryout begins. The predictive accuracy of five classical two-phase heat transfer correlations for miniature channel flow is examined. Selecting the best fitting of the classical ...
92 citations
TL;DR: In this article, the authors classified the cold plates into four types: formed tube, formed tube formed tube (STB), formed tube and formed tube tube (SCR) cold plates.
Abstract: Electronics cooling research has been largely focused on high heat flux removal from computer chips in the recent years. However, the equally important field of high-power electronic devices has been experiencing a major paradigm shift from air cooling to liquid cooling over the last decade. For example, multiple 250-W insulated-gate bipolar transistors used in a power drive for a 7000-HP motor used in pumping or in locomotive traction devices would not be sufficiently cooled with air-cooling techniques. Another example is a “hockey puck” SCR of 63 mm diameter used to drive an electric motor that could dissipate over 1500 W and is difficult to cool with air because of the shape of the device. Other devices include radio-frequency generators, industrial battery chargers, printing press thermal and humidity control equipment, traction devices, mining devices, crude oil extraction equipment, magnetic resonance imaging, and railroad engines. This article classifies the cold plates into four types: formed tube...
79 citations
TL;DR: In this article, the effect of nanoparticles on droplet boil-off was studied and the working fluids used were pure water, ethanol, and dimethyl sulfoxide (DMSO) and ethanol or DMSO-nanoparticle solutions (the nanoparticles were aluminum, with concen...
Abstract: Spray cooling is used in cooling of electronic devices to remove large heat fluxes. Heat transfer to droplets impinging on a heated surface and boiling off has been studied. Most work is on a well-controlled system of a single drop falling onto a horizontal heated plate from a fixed height. These have revealed the droplet impingement mechanics to be a function largely of Weber number and excess temperature, and a range of regimes is observed similar to those in pool boiling, with a clearly identifiable critical heat flux. Nanofluids exhibit enhanced boiling heat transfer in pool boiling. The effect of nanoparticles on droplet boil-off was studied in this work. Nanofluid drops were let fall onto a surface at temperature greater than the saturation temperature, and behavior and heat flux were recorded and contrasted to that of a pure fluid. The working fluids used were pure water, ethanol, and dimethyl sulfoxide (DMSO) and ethanol– or DMSO–nanoparticle solutions (the nanoparticles were aluminum, with concen...
76 citations
TL;DR: In this paper, the authors discuss the problem of fouling of heat exchangers in processing industries, and propose a method to mitigate the fouling in the process of heat exchange.
Abstract: Fouling of heat exchangers in processing industries is a chronic operational problem that compromises energy recovery and environmental welfare. Mitigation of fouling is frequently restricted by th...
72 citations
TL;DR: In this article, the air-side heat transfer and friction characteristics of nine kinds of fin-and-tube heat exchangers, with a large number of tube rows (6, 9, and 12, respectively) and a large diameter of tubes (18 mm), are experimentally investigated.
Abstract: Air-side heat transfer and friction characteristics of nine kinds of fin-and-tube heat exchangers, with a large number of tube rows (6, 9, and 12, respectively) and large diameter of tubes (18 mm), are experimentally investigated The test samples consist of three types of fin configurations: plain fin, slit fin, and fin with delta-wing longitudinal vortex generators The working fluid in the tube is steam Results show that when the number of tube is larger than 6, the heat transfer and friction performance for three kinds of fins is independent of the number of tube rows, and slit fin provides higher heat transfer and pressure drop than the other two fins The heat transfer and friction factor correlations for all the heat exchangers were acquired with Reynolds numbers ranging from 4000 to 10000 The air-side performance of heat exchangers with plain fin, slit fin, and longitudinal vortex-generator fin were evaluated under three sets of criteria, and the results showed that the heat exchanger with slit
67 citations
TL;DR: In this paper, the initial fouling rates of four crude oils were determined at a nominal bulk temperature of 315°C, an initial heated wall shear stress of 13 Pa, and initial surface temperatures between 375 and 445°C.
Abstract: The initial fouling rates of four crude oils were determined at a nominal bulk temperature of 315°C, an initial heated wall shear stress of 13 Pa, and initial surface temperatures between 375 and 445°C. These initial fouling rates ranged from 1.3(10− 6) to 7.8(10− 5) m2 K/kJ. Corresponding Arrhenius plots were linear with the initial fouling rates passing through an isokinetic temperature of 407.5°C. A plot of the natural logarithm of the pre-exponential factors [7.6(104)–5.2(1015) m2 K/kJ] versus the apparent activation energies (128–269 kJ/mol) was also linear, confirming the validity of the isokinetic temperature and the presence of the compensation effect. Below the isokinetic temperature, the relative fouling rates were Crude Oil C > Crude Oil A > Crude Oil D > Crude Oil B; above the isokinetic temperature, the relative fouling rates were reversed (Crude Oil B > Crude Oil D > Crude Oil A > Crude Oil C). Chemical characterization of a fouling deposit suggested that the dominant fouling mechanism at th...
53 citations
TL;DR: In this paper, a new approach for energy and exergy efficiencies and effectiveness of latent heat storage systems is developed and applied to a shell-and-tube type LSH system for its charging process.
Abstract: In this article, a new approach for energy and exergy efficiencies and effectiveness of latent heat storage systems is developed and applied to a shell-and-tube type latent heat storage system for its charging process. First, a numerical model is introduced to study the heat transfer and thermodynamic parameters and aspects (shell radius and pipe length, Reynolds number, inlet temperature of heat transfer fluid [HTF], etc.), and solved through the governing equations for HTF, pipe wall, and phase change material. Second, some extensive parametric studies are conducted to investigate how the solidification fronts, stored heat, heat transfer rates, entropy generation number, and both energy and exergy efficiencies change with time, particularly in the dimensionless form as the Fourier number. Third, both energetic and exergetic effectiveness concepts are newly developed and applied to a latent heat storage system. The results show that energy and exergy efficiency and effectiveness concepts appear to be sig...
TL;DR: In this paper, different surface coatings on stainless steel were investigated with respect to fouling tendency, and the effect of different surface crystallization due to energetic modification is reduced because of the dominating effect of the low adhesive surface.
Abstract: To minimize the negative effects of scale formation in heat exchangers, new anti-fouling strategies are focusing on the modification of heat transfer surfaces. These modifications should lead to tailor-made surfaces for different technical applications. The aim of this surface modification is the extension of the induction period to minimize the negative effects of fouling and maximize the endurance of the heat exchanger. To achieve this, different surface coatings on stainless steel were investigated with respect to fouling tendency. The effects of flow velocity with respect to Reynolds number on the induction time of CaSO4 crystallization fouling were tested in different test units. Diamond-like carbon (DLC) coatings extend the induction time at every measured flow velocity. At higher Reynolds numbers, the effect of different surface crystallization due to energetic modification is reduced because of the dominating effect of the low adhesive surface. Thus the induction time can be extended by the factor...
TL;DR: In this article, a package-level numerical simulation is developed to predict the on-chip hot spot cooling capability achievable with such a mini-contact enhanced TEC, focusing on the hot-spot temperature reduction associated with variations in mini contact size and thermoelectric element height.
Abstract: Shrinking feature size and increasing transistor density, combined with the high performance demanded from next-generation microprocessors, have led to on-chip high heat flux “hot spots,” which have emerged as the primary driver for thermal management of today's integrated circuit (IC) technology. This article describes the use of a mini-contact to enhance the cooling flux of a miniaturized thermoelectric cooler (TEC) for on-chip hot-spot remediation. A package-level numerical simulation is developed to predict the on-chip hot spot cooling capability achievable with such a mini-contact enhanced TEC. Attention is focused on the hot-spot temperature reduction associated with variations in mini-contact size and thermoelectric element height, as well as the parasitic effect from the thermal contact resistance introduced by the mini-contact. A preliminary experiment has been conducted to verify the numeric model and to demonstrate the effects of the mini-contact on hot-spot cooling.
TL;DR: In this article, a functional relationship of the Nusselt number is established for arrays of round impinging jets, and a new, simpler correlation is developed that better represents experimental data published by several researchers.
Abstract: Jet impingement heat transfer has received a remarkable amount of attention by researchers throughout the world for many years due to widespread interest from both academic and industrial points of view. However, the fluid dynamics involved with arrays of jets are very complex and still not completely understood. Therefore, a functional relationship of the Nusselt number, which involves thermophysical and dynamic quantities as well as geometric parameters, is still not univocally and generally established. In this research analysis, some empirical equations, which are available in the literature for arrays of round impinging jets, are reviewed, and a new, simpler correlation is developed that better represents experimental data published by several researchers.
TL;DR: In this article, a novel solar thermal power plant with a floating chimney stiffened on a mountainside segment by segment is proposed for the special topography in China (i.e., a vast desert belt surrounded by high mountain chains up to thousands of meters).
Abstract: A novel solar thermal power plant with a floating chimney stiffened on a mountainside segment by segment is proposed. The novel power plant is suitable for the special topography in China (i.e., a vast desert belt surrounded by high mountain chains up to thousands of meters). An investigation of its performance is carried out using a simple mathematical model. The levelized electricity cost and the potential of the proposed solar chimney power generation in the large desert regions in Northwest China are also estimated. The results show that the levelized electricity cost of the proposed power generation is competitive with that of clean coal power plant, that the potential power obtained from the proposed power plant in the Taklamakan Desert or the Badain Jaran Desert can satisfy the total electricity consumption in China, and that the total potential power in the twelve deserts and sands, reaching 25,761 TWh per year, can even supply the electric power needs over the world, which are rapidly increasing.
TL;DR: In this article, a 3D pin-fin tube was used for condensation of steam at atmospheric pressure and low velocity on five three-dimensional integral-fin tubes, and the main geometric parameters varied were the circumferential pin spacing and thickness, since these have been shown to have a strong effect on condensate retention.
Abstract: Experimental data are reported for condensation of steam at atmospheric pressure and low velocity on five three-dimensional pin-fin tubes. The main geometric parameters varied were the circumferential pin spacing and thickness, since these have been shown to have a strong effect on condensate retention, and the present study shows some evidence for an optimum circumferential fin spacing. Enhancements of the vapor-side heat-transfer coefficient of up to 4 were found, compared to a plain tube at the same vapor-side temperature difference. The measured enhancements are equal to, but do not exceed, those obtained from “optimized” two-dimensional integral-fin tubes reported in the literature—an observation that is also generally true for condensation of refrigerants. The evidence suggests, however, that three-dimensional fin profiles can produce worthwhile enhancement over those obtained from simple, two-dimensional, integral-fin tubes, but that more work is needed to understand the phenomena involved so that ...
TL;DR: In this article, an experimentally validated numerical model is developed to analyze the operation of a piezoelectrically actuated cantilever vibrating close to a heated surface.
Abstract: An experimentally validated numerical model is developed to analyze the operation of a piezoelectrically actuated cantilever vibrating close to a heated surface. The vibrating cantilever acts as a fan and provides localized cooling. The numerical results for the flow field and heat transfer show satisfactory agreement with experiments. The numerical model is used to develop fan curves for the piezoelectric fans, using a methodology similar to that used in constructing pump or fan curves for conventional fans. A simplified model based on stagnation region heat transfer in impingement flows is also proposed to estimate the heat transfer from a piezoelectric fan. The velocities obtained from the piezoelectric fan curves generated are used in this impingement heat transfer model, and the predictions are found to agree with measured stagnation zone Nusselt numbers with an average deviation of 17%.
TL;DR: In this article, the effects of the inclination angle, filling ratio, input heat rate, mass flowrate of coolant, and inside diameter of the tube on the geyser boiling phenomenon have been investigated experimentally.
Abstract: In this paper, geyser boiling phenomenon (GBP) in a two-phase closed thermosyphon has been investigated experimentally. Here, the effects of the inclination angle, filling ratio, input heat rate, mass flowrate of coolant, and inside diameter of the tube on the GBP have been discussed. Three copper thermosyphons with inside diameters of 14 mm, 20 mm, and 24 mm and a length of 1000 mm were employed. Distilled water was used as the working fluid. A series of experiments was carried out to investigate the effect of the inclination angle range of 5° to 90°, the input heat rate range of 50 to 312.4 W, the coolant mass flow rate range of 0.00389 to 0.0164 kg/s, and the filling ratio range of 15 to 45%. The GBP has been investigated by analyzing the time variations of the evaporator and adiabatic wall temperature and outlet water temperature from condenser jacket. The results show that the period of GBP was longer for higher inclination angles and filling ratios. Furthermore, it was discovered that the GBP did no...
TL;DR: In this article, the authors introduce a novel manufacturing technique, selective laser melting (SLM), and demonstrate its ability to fabricate new designs of heat sink that have not previously been considered, primarily due to their geometric complexity.
Abstract: Heat removal from electronic packages is often assisted with the use of heat sinks whose heat transfer surfaces come in a variety of forms such as cylindrical pins, flat fins, and corrugated sheet. These conventional designs are manufactured by traditional methods such as forging, machining, casting, stamping and bending, or a combination of processes. This article introduces a novel manufacturing technique, selective laser melting (SLM), and demonstrates its ability to fabricate new designs of heat sink that have not previously been considered, primarily due to their geometric complexity. Three novel finned structures have been manufactured and their thermal and fluid flow characteristics have been determined experimentally. The three heat sinks demonstrate selective laser melting's ability to produce fine detail and consist of a staggered elliptical array, an elliptical array where the pins are angled in a direction perpendicular to the flow, and a densely packed diamond array. The novel heat sink desig...
TL;DR: In this paper, a robust and reliable simulator, reliable fouling models, and the ability to handle the thermal and hydraulic impacts of fouling was proposed for refinery preheat trains.
Abstract: Optimizing cleaning schedules for refinery preheat trains requires a robust and reliable simulator, reliable fouling models, and the ability to handle the thermal and hydraulic impacts of fouling. The interaction between thermal and hydraulic effects is explored using engineering analyses and fouling rate laws based on the “threshold fouling” concept; the potential occurrence of a new phenomenon, “thermo-hydraulic channeling” in parallel heat exchangers, is identified. The importance of the foulant thermal conductivity is highlighted. We also report the development of a highly flexible preheat train simulator constructed in MATLAB/Excel. It is able to accommodate variable throughput, control valve operation, and different cost scenarios. The simulator is demonstrated on a network of 14 heat exchangers, where the importance of optimizing the flow split between parallel streams is illustrated.
TL;DR: In this paper, the authors present an experimental study of the airflow inside a solar chimney, which uses solar energy to generate hot airflow that can be used to dry agricultural products or generate electric power.
Abstract: The development of alternative energy sources is increasingly becoming a necessity. In this context, solar energy stands out because it is a renewable and low pollutant source of energy. The solar chimney uses solar energy to generate hot airflow that can be used to dry agricultural products or generate electric power. This paper presents an experimental study of the airflow inside a solar chimney. The tests were performed in a prototype built on the campus of Universidade Federal de Minas Gerais, with appropriate dimensions to dry agricultural products. The values of temperature, velocity, solar radiation, and humidity inside and outside the device were monitored over a nine month period, allowing the characterization of the airflow and the determination of the optical properties of the materials used.
TL;DR: In this article, the authors developed a concept for a suitable startup condition for a closed-loop oscillating heat pipe (CLOHP) by using visual data and the thermodynamics theory for predicting the amount of vapor evaporation and condensation in a CLOHP.
Abstract: This article develops a concept for a suitable startup condition for a closed-loop oscillating heat pipe (CLOHP). This concept was developed by using visual data and the thermodynamics theory for predicting the amount of vapor evaporation and condensation in a CLOHP. The visual data indicated that the key to a suitable startup is the amount of net vapor expansion in the evaporator and the amount of net collapsed vapor in the condenser. Initial dryout, an event that occurs after a startup failure, results when the net vapor expansion is higher than the amount of net vapor collapsed. This situation obstructs the replacement process. This is a mechanism in which the volume of mixture from the condenser section flows to the evaporator section to replace the volume of mixture that leaves the evaporator section. When the replacement process is impeded, all of the liquid in the evaporator section evaporates and the evaporator section is not refilled by the mixture from the condenser section. The evaporator secti...
TL;DR: In this article, an accurate numerical model including 3D steady Navier-Stokes equations in Cartesian coordinate system is developed to evaluate the performance of a compressible flow through a solar chimney.
Abstract: An accurate numerical model including three-dimensional (3D) steady Navier–Stokes equations in Cartesian coordinate system is developed to evaluate the performance of a compressible flow through a solar chimney. This model is validated by comparison with the previous numerical simulated results for one-dimensional compressible flow in a proposed 1500-m-high solar chimney with an inner diameter of 160 m. Results for this 3D numerical model of a compressible flow through a solar chimney are more accurate compared to those from the Boussinesq model and full-buoyancy model. Normally, an incompressible flow is considered in the Boussinesq model, and enthalpy loss is not included in the energy equation in both the Boussinesq model and full-buoyancy model. Moreover, buoyancy reference density is kept as constant in the energy equation in the full-buoyancy model. The results show that the use of the Boussinesq model and full-buoyancy model with some commercial numerical computational fluid dynamics programs usual...
TL;DR: In this article, an experimental investigation is reported for medium-temperature, wickless, small-diameter heat pipes charged with environmentally sound and commercially available working fluids, and the working fluids tested were diphenyl based: Therminol VP1 and Dowtherm A.
Abstract: An experimental investigation is reported for medium-temperature, wickless, small-diameter heat pipes charged with environmentally sound and commercially available working fluids. The wickless heat pipes (thermosyphons) studied have many applications in heat recovery systems since their operational temperature range is between 200°C and 450°C. The heat pipes investigated had an internal diameter of 6 mm and a length of 209 mm. The lengths of evaporator and condenser sections were 50 mm and 100 mm, respectively. The working fluids tested were diphenyl based: Therminol VP1 and Dowtherm A. High-grade stainless steel was chosen as the shell material for the heat pipes to provide chemical compatibility between heat pipe casing material and working fluids at elevated temperatures. Thermal resistances of less than 0.4 K/W have been achieved at working temperatures of up to 420°C with an effective thermal conductivity of 20 kW/m-K, which corresponds to an axial heat flux of 2.5 MW/m2. Even for such small-diameter...
TL;DR: In this paper, the effect of various parameters on the maximum heat flux of a vertical closed-loop pulsating heat pipe (CLPHP) and the inside phenomena that cause maximum heat flow to occur were investigated.
Abstract: The objective of this study is to experimentally investigate the effect of various parameters on the maximum heat flux of a vertical closed-loop pulsating heat pipe (CLPHP) and the inside phenomena that cause maximum heat flux to occur. A correlation to predict the maximum heat flux using the obtained results was also established. Quantitative and qualitative experiments were conducted and analyzed. A copper CLPHP and a transparent high-temperature glass capillary tube CLPHP were used in the quantitative and qualitative experiments. From the study, it was found that when the internal diameter and number of meandering turns increased, the maximum heat flux increased. However, when the evaporator section length increased, the maximum heat flux decreased. The maximum heat flux of a CLPHP occurs due to the dry-out of liquid film at the evaporator section. This occurs after a two-phase working fluid circulation changes flow pattern from countercurrent slug flow to co-current annular flow, because the vapor vel...
TL;DR: In this paper, the influence of flow direction with respect to gravity on particulate fouling of heat exchangers is investigated experimentally to determine the optimal flow direction to minimize fouling.
Abstract: The influence of flow direction with respect to gravity on particulate fouling of heat exchangers is investigated experimentally to determine the optimal flow direction to minimize fouling. Four orientations of flow have been investigated: horizontal flow, upward flow, downward flow, and a flow under an angle of 45°. It is observed that fouling starts at the point of stagnation irrespective of the flow direction, and also at the top of the heat exchanger tubes. Particulate fouling grows from these two points till they meet and the fouling layer covers the whole surface of the heat exchanger tube. Fouling at the upper half of the tubes is much faster than the lower half of the tubes, and the fouling rate is faster at the bottom tubes of the heat exchanger section than at the upper tubes. The best orientation for lingering particulate fouling is the downward flow, where the flow stagnation point coincides with the top point of the heat exchanger tubes and the growth of the fouling layer only starts from one...
TL;DR: In this paper, a review brings out those aspects of the development of Proton Exchange Membrane (PEM) fuel cells over the last two to three decades that are of interest to the heat and mass transfer community.
Abstract: This review brings out those aspects of the development of proton exchange membrane (PEM) fuel cells over the last two to three decades that are of interest to the heat and mass transfer community. Because the heat transport and mass transport in proton exchange membrane fuel cells are very important from the efficiency point of view, an emphasis is given here to these transports and their influence on operating cell parameters. The works are classified as models with either isothermal or non-isothermal conditions of various assumed dimensionality and with either single-phase or two-phase flow. Along with modeling, a few experimental studies available are also reported here. Researchers in the area of PEM fuel cells are involved in activities such as development of new and low-cost materials, modeling the relevant physical processes, and electrochemical experimentation. These collective efforts may lead to making this technology viable to meet world needs for clean and cheap energy. This review brings out...
TL;DR: In this article, a brief summary of the development of the vehicular A/C system is provided, with historical development from the inception of the A-C system to future systems: R12, R134a, and enhanced R 134a.
Abstract: Automotive air-conditioning (A/C) or mobile air-conditioning (MAC) systems have played an important role in human comfort and to some extent in human safety during vehicle driving in varied atmospheric conditions. It has become an essential part of the vehicles of all categories worldwide. After discussing the basic operation of the A/C system, a brief summary is provided on historical development of the vehicular A/C system, with refrigerant history from the inception of the A/C system to future systems: R12, R134a, and enhanced R134a A/C system, and next-generation refrigerants having no ozone depletion potential in the stratosphere and global warming potential less than 150. The discussion also includes an enhanced MAC system with R134a, and the direct and indirect emissions from vehicles impacting global warming due to the use of the A/C system. This would explain why we continue to change the refrigerants in the automotive A/C system in spite of billions of dollars of cost for the previous refrigeran...
TL;DR: In this article, a linear dependency of the logarithm of the preexponential factor on apparent activation energy for the crude oil was found in a model chemical system of styrene polymerization.
Abstract: Maya crude oil fouling reveals a seemingly straightforward dependency of initial fouling rate on surface temperature, but a maximum is found in the initial fouling rate–velocity relationship, which mirrors that found in a model chemical system of styrene polymerization. The linear dependency of the logarithm of the pre-exponential factor on apparent activation energy for the crude oil is also found in the styrene system. The apparent activation energy for the crude oil ranged from 26.4 kJ/mol at 1.0 m/s to 245 kJ/mol at 4.0 m/s. Such strong dependencies of apparent activation energy on velocity, even at high velocity, are consistent with Epstein's mass transfer reaction attachment model. Surface temperatures at which the fouling rate becomes velocity independent are 274°C and 77°C for Maya crude oil and styrene, respectively. For surface temperatures in excess of this isokinetic temperature, an increase in velocity would lead to an increase in the rate of fouling.
TL;DR: In this paper, heavy oil fractions rich in asphaltenes were mixed with diluents containing from 0.6 to 25% aromatics, and the resulting blends subjected to batch precipitation experiments at 85°C, and thermal fouling tests at surface temperatures in the range 230-310°C.
Abstract: Heavy oil fractions rich in asphaltenes were mixed with diluents containing from 0.6 to 25% aromatics, and the resulting blends subjected to batch precipitation experiments at 85°C, and thermal fouling tests at surface temperatures in the range 230–310°C. Deposit compositions were determined, and are compared with suspended asphaltene composition. When the heavy oils were blended with the most aromatic diluents, precipitation and fouling were negligible. As the aromaticity of the diluents was decreased, the extent of asphaltene precipitation and the fouling rates increased. The solubility parameter of the blends, δ mix , and the flocculation solubility parameter for asphaltenes, δ f , were determined from measurements of the asphaltene flocculation onset by titration with heptane at temperatures from 25 to 50°C. Literature models predict no asphaltene precipitation (and presumably little fouling) will occur when [δ mix – δ f ] > 0. Both the amount of asphaltene precipitated in the batch experiments and th...