Oil fouling in double‐pipe heat exchanger under liquid‐liquid dispersion and the influence of copper oxide nanofluid

Dispersions of oil in water are encountered in a variety of industrial processes leading to a reduction in the performance of the heat exchangers when thermally treating such two phase fluids. This reduction is mainly due to changes in the thermal and hydrodynamical behavior of the two phase fluid. In the present work, an experimental investigation was performed to study the effects of light oil fouling on the heat transfer coefficient in a double‐pipe heat exchanger under turbulent flow conditions. The effects of different operating conditions on the fouling rate were investigated including: hot fluid Reynolds number (the dispersion), cold fluid Reynolds number, and time. The oil fouling rate was analyzed by determining the growth of fouling resistance with time and through pressure drop measurements. The influence of copper oxide (CuO) nanofluid on the fouling rate in the dispersion was also determined. It was found that the presence of dispersed oil causes a reduction in heat transfer coefficient by percentages depending on the Reynolds number of both cold and hot fluids and the concentration of oil. In addition, the time history of fouling resistance exhibited different trends with the flow rates of both fluids and its trend was influenced appreciably by the presence of CuO nanofluid.     

Long time investigation of the effect of fouling on the super‐heaters in a circulating fluidized biomass boiler

The present investigation involves measurements and theories on the mechanisms of the forming of deposit layers on super‐heater tubes in a biomass‐fired CFD boiler. The deposit layer thickness and the soot‐blowing frequency effect on the super‐heaters heat transfer are the main subject of the study that has been conducted over a 3‐year period. The measurements show a deposit growth rate on the super‐heaters of approximately 4 g m^?2 h^?1. The distribution of the deposit material varies significantly between the windward and the leeward side of the tubes, with the thickest layers on the windward side. Further down stream of the first super‐heater, the fouling problem on the super‐heater and re‐heater tubes are not so severe. A theoretical model shows that a deposit layer of 20 mm will decrease the heat transfer rate of the first super‐heater by nearly 40%. The soot‐blowing system shows a strong positive effect on the heat transfer rate of the super‐heater a few hours after a soot‐blowing sequence has been completed. However in the long run, the varied soot‐blowing frequency does not have a significant influence on the deposit layer growth rate. Copyright © 2006 John Wiley & Sons, Ltd.     

Impact of scale build‐up models on energy utilization in heat exchangers

In this paper an energy utilization index is presented. It is used to investigate the impact of scale build‐up in heat exchange equipment. The investigation is carried out using several scale build‐up models. The models employed in this paper are the linear, logarithmic, and the asymptotic one. It has been demonstrated by a realistic example that the type of scale build‐up model assumed greatly affects energy utilization and maintenance scheduling for heat exchange equipment. Copyright © 1999 John Wiley & Sons, Ltd.     

Modelling and experimental studies on heat transfer in the convection section of a biomass boiler

This paper describes a model of heat transfer for the convection section of a biomass boiler. The predictions obtained with the model are compared to the measurement results from two boilers, a 50 kW_th pellet boiler and a 4000 kW_th wood chips boiler. An adequate accuracy was achieved on the wood chips boiler. As for the pellet boiler, the calculated and measured heat transfer rates differed more than expected on the basis of the inaccuracies in correlation reported in the literature. The most uncertain aspect of the model was assumed to be the correlation equation of the entrance region. Hence, the model was adjusted to improve the correlation. As a result of this, a high degree of accuracy was also obtained with the pellet boiler. The next step was to analyse the effect of design and the operating parameters on the pellet boiler. Firstly, the portion of radiation was established at 3–13 per cent, and the portion of entrance region at 39–52 per cent of the entire heat transfer rate under typical operating conditions. The effect of natural convection was small. Secondly, the heat transfer rate seemed to increase when dividing the convection section into more passes, even when the heat transfer surface area remained constant. This is because the effect of the entrance region is recurrent. Thirdly, when using smaller tube diameters the heat transfer area is more energy‐efficient, even when the bulk velocity of the flow remains constant. Copyright © 2006 John Wiley & Sons, Ltd.     

Numerical simulation of separated flow transition and heat transfer around a two‐dimensional rib

Numerical simulations of separated flow transition and heat transfer around a two‐dimensional rib mounted in a laminar boundary layer were performed. The separated shear layer becomes unstable due to the Kelvin–Helmholtz instability and generates a two‐dimensional vortex. This vortex becomes three‐dimensional and collapses in the downstream part of the separation bubble. As a result, transition from laminar to turbulent flow occurs in the separated shear layer. Streamwise vortices exist downstream of the reattachment flow region. The low‐frequency flapping motion and transition of the separated shear layer are influenced by three‐dimensional dynamics upstream of the separation bubble. Large‐scale vortices around the reattachment flow region have substantial effects on heat transfer. Downstream of the reattachment point, the surface friction coefficient and Nusselt number are different from their profiles in the laminar boundary layer and approach the distributions seen in the turbulent boundary layer. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(8): 513–528, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20177     

Heat transfer for flow boiling of water and critical heat flux in a half‐heated round tube under low‐pressure conditions

Heat transfer for flow boiling of water and critical heat flux (CHF) experiments in a half‐circumferentially heated round tube under low‐pressure conditions were carried out. To clarify the flow patterns in the heated section, experiments in the round tube under the same conditions were also carried out, and their results were compared. The experiments were conducted with atmospheric‐pressure water in test sections with inner diameter D = 6 mm, heated length L = 360 mm, inlet water subcooling ΔT_in = 80 K, and mass velocity G from 0 to 2000 kg/(m²·s) for the half‐circumferentially heated round tube and from 0 to 7000 kg/(m²·s) for the full‐circumferentially heated tube. The experimental data demonstrated that the wall temperature near the outlet of the half‐circumferentially heated tube remained almost the same until CHF. It was found that burnout occurred when the flow regime changed from churn flow to annular flow, and the liquid film on the heated wall dried out although liquid film on the unheated wall remained. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(3): 149–164, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10022     

Prediction of local heat transfer characteristics of dilute gas‐solid flows through an adiabatic,horizontal pipe

The local heat transfer characteristics of gas‐solid flows through an adiabatic, horizontal pipe are numerically studied using the two‐fluid model of Ansys Fluent 15. First, the model is validated with the experimental results available in the literature for the air temperature and average Nusselt number. Then, the local heat transfer characteristics of gas‐solid flows, such as temperature profiles of gas and solid, gas‐solid Nusselt number, logarithmic mean temperature difference, and effectiveness of gas and solid, are studied by changing different parameters (gas velocities 15‐24 m/s; inlet solid loading ratios 0.1‐1; particle diameters 100‐400 µm). It is observed that increasing the particle diameter and inlet gas velocity increases the gas temperature and decreases the solid temperature, increases the logarithmic mean temperature difference, and decreases the thermal effectiveness of gas and solid. However, increasing the solid loading ratio decreases the gas and solid temperatures, decreases the logarithmic mean temperature difference, and increases the thermal effectiveness of gas and decreases the thermal effectiveness of solid. Moreover, increasing the particle diameter decreases the gas‐solid Nusselt number, whereas increasing the solid loading ratio and inlet gas velocity increase the gas‐solid Nusselt number.     

Investigation on the heat transfer characteristics of propylene glycol–water mixture in the shell side of a spiral‐wound heat exchanger

The heat transfer characteristics of propylene glycol–water (PG–W) mixture (10%, 20%, and 30% propylene glycol) on the shell side of a spiral‐wound heat exchanger (SWHE) were investigated experimentally. Among the SWHE selected, there are 18 twined tubes with a diameter of 8 mm. PG–W mixture is on the shell side and water is on the tube side. The results show that the heat transfer coefficient of PG–W mixture flowing downwards is higher than upwards under countercurrent conditions. The heat transfer coefficient decreases with the increasing of concentration of PG–W mixture. When the inclination angle of the SWHE is 90°, the heat transfer coefficient of PG–W mixture is the largest; and when the inclination angle is less than 90°, the heat transfer coefficient decreases with the decrease of inclination angle. The inclination angle has a great effect on the heat transfer coefficient at a high concentration. The fitting correlation equations between Nu, Re, Pr, and inclination angles of SWHE are established.     

Heat transfer enhancement by using a rectangular wing vortex generator on the triangular shaped fins of a plate‐fin heat exchanger

The present numerical analysis pertains to the heat transfer enhancement in a plate‐fin heat exchanger employing triangular shaped fins with a rectangular wing vortex generator on its slant surfaces. The study has been carried out for three different angles of attack of the wing, i.e., 15°, 20° and 26°. The aspect ratio of the wing is not varied with its angle of attack. The flow considered herein is laminar, incompressible, and viscous with the Reynolds number not exceeding 200. The pressure and the velocity components are obtained by solving the continuity and the Navier– Stokes equations by the Marker and Cell method. The present analysis reveals that the use of a rectangular wing vortex generator at an attack angle of 26° results in about a 35% increase in the combined spanwise average Nusselt number as compared to the plate‐triangular fin heat exchanger without any vortex generator. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20285     

Study on high performance evaporator with micro‐grooves (Measurement of capillary force in a single groove and modeling of heat and mass transfer)

A micro‐grooved evaporator is composed of µm‐wide grooves on a heat transfer plate in which the inter‐line regions at the liquid–vapor meniscus of coolant become identifiable. The high‐heat performance of the evaporator is realized by this inter‐line region (ILR) where the liquid thin film reduces the thermal resistance on the heat transfer surface. In this report, we propose a numerical simulation model of heat and mass transfer in a single groove to predict its capillary force and heat flux. The capillary force performance (capillary‐rise length in a groove) of a single groove was measured for samples of varying width, superheat, and inclination. The performance was found to be a maximum at a specific groove width of 200–400 µm, which is in good agreement with the predicted results calculated by the proposed model. For a better prediction of capillary‐rise length, the effective capillary force and the effective flow resistance were considered. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20257     

Effects of operating conditions on infiltration of molten aluminum and heat transfer in a centrifugal force field

This paper presents the results of an analysis aimed at determining the influence of changing operating conditions in the centrifugal infiltration casting. It considers the effect of centrifugal force on infiltration and heat transfer. The molten aluminum flow with heat transfer though SiC porous media in a centrifugal force field is described using a mathematical and physical model by employing the local thermal nonequilibrium between the solid and fluid phases. The calculation results show that the temperature difference between molten aluminum and SiC porous media in the infiltrated region decreases with the contact time. There are two distinctly noticeable stages of infiltration velocity: the onset stage of infiltration, which drops down sharply, and the following stage of smooth velocity. The operating conditions have important effects on the infiltration velocity and temperature patterns of fluid and solid. A suitable rotational speed and SiC volume fraction should be chosen to ensure the flow of molten metal in the porous preform and diminish the temperature difference between fluid and solid. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(6): 501–510, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10114     

Characteristics of heat transfer inside a tube during sodium‐water reaction in a FBR steam generator

Sodium reacts chemically with water in the case of an unexpected tube failure of a steam generator (SG) in a fast breeder reactor (FBR). In order to predict the event with high accuracy, it is very important to understand the characteristics of heat transfer inside the tube in detail during the tube failure due to the sodium–water reaction. Experiments were performed by using purified water under the following conditions: initial pressure of 11.2–13.4 MPa, initial water temperature of 200 °C, and water mass flux of 45.7 to 3630 kg/(m²s). The test tube was heated rapidly by high‐frequency induction current. The time averaged heat flux was estimated by using an inverse solution from the measured temperatures at two points on three different locations along the tube. It was confirmed that the derived values agreed with the measured heat fluxes on the outer surface within 20% accuracy. It was found that the characteristics of the heat transfer strongly depend on the flow rate. The heat transfer on the wall changed from nucleate boiling to transient‐film boiling during increasing the heat flux and returned to the nucleate boiling during decreasing the heat flux. A counterclockwise cycle always appeared in the transition boiling region, where the nucleate and film boiling coexisted and the area ratio of these varied with time. The adequacy of heat transfer correlations to evaluate tube overheating was confirmed. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20320     

Local instantaneous temperature and time‐averaged heat transfer coefficient in the bottom zone of a circulating fluidized bed

Local instantaneous temperature signal and time‐averaged heat transfer coefficient were measured using a miniature heat transfer probe. The experiments were carried out in the bottom zone of a 5.8m high, 0.3m×0.5m rectangular cross‐section circulating fluidized bed. The results show that the heat transfer coefficient was higher near the walls, and became lower near the central region, and that the heat transfer coefficient decreases with increment of the air velocity due to the associated reduction of solids holdup in the bottom zone. In addition, the power spectrum density functions of the local instantaneous temperature signal can be characterized by the 1/f‐like distribution. Copyright © 2004 John Wiley & Sons, Ltd.     

Effect of fins on forced convection heat transfer around a tube in an aligned‐arranged tube bundle

The effect of fins on heat transfer around a tube in an aligned‐arranged tube bundle was investigated experimentally, and the obtained results were compared for three arrangements, i.e., single tube, single tube row, and staggered‐arrangement. It was found from the experiment that the effect of fins begins to appear in an aligned‐arrangement with larger fin spacing than in a staggered‐arrangement. The degradation in the local heat transfer coefficient due to fins can be recognized not only on the rear region of the tube, as observed in other arrangements, but also on the frontal region. As a result of this phenomenon, the degradation in the average heat transfer coefficient in an aligned‐arrangement becomes larger than in other arrangements with the same fin spacing. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(8): 555–563, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20091     

Heat transfer characteristics of a reservoir embedded loop heat pipe (Heat transfer characteristics of a deployable radiator for use on the ETS‐VIII satellite while in orbit)

As heat generation in satellites increases, ensuring that they are provided with sufficient radiator panel area is an important problem. Deployable radiators, with radiator panels that are deployed post‐launch in space to increase the satellite's effective radiator panel area of the satellite, are becoming an important thermal control technology. A reservoir embedded loop heat pipe (RELHP) is used in deployable radiators as a heat transport device. A deployable radiator of this type was mounted on the ETS‐VIII satellite, which was launched on December 18, 2006 and injected into a geostationary orbit. The satellite is still operating without any significant issues over two years later. This paper investigates the heat transport characteristics of an RELHP system used in a deployable radiator in a geostationary orbital environment. This system can be successfully started up in a micro gravity environment. We also found that the sub‐cooling region is shorter in a micro gravity environment than in a terrestrial gravity environment, because there is less heat leakage into the reservoir in a micro gravity environment. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20346     

Effect of double stratification on mixed convection heat and mass transfer from a vertical surface in a fluid‐saturated porous medium

This paper presents the mixed convection heat and mass transfer near a vertical surface in a stratified porous medium using an integral method. The conservation equations that govern the problem are reduced to a system of coupled non‐linear ordinary differential equations, which is then reduced into a single algebraic equation using exponential profiles for the temperature and concentration. The results for heat and mass transfer rates in terms of Nusselt and Sherwood number are presented for a wide range of governing parameters like the buoyancy ratio (N), Lewis number (Le), flow driving parameter (Ra/Pe), in addition to both thermal and solutal parameters (S and R). The results indicate that the stratification effects have considerable influence on both the heat and mass transfer rates. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20300     

Application of radially varying magnetic field on a peristaltic flow of non‐Newtonian fluid in the presence of heat and mass transfer

This article deals with the variable MHD effects on the peristaltic flow of a non‐Newtonian fluid in the presence of heat and mass transfer. The walls of annulus are maintained at different temperatures. Continuity, momentum concentration, and energy equations are utilized in the mathematical analysis. Two types of solutions, namely, the exact and numerical, are derived. These solutions are compared and discussed. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20314     

Mixed convective heat and mass transfer on a peristaltic flow of a non‐Newtonian fluid in a vertical asymmetric channel

In the present analysis we discuss the effects of mixed convective heat and mass transfer on the peristaltic flow of a non‐Newtonian fluid in a vertical asymmetric channel. The flow is investigated in a wave frame of reference moving with the velocity c away from the fixed frame. The governing equations for the present flow problem are first modeled and then discussed. The analytical solution of the present flow problem is discussed using regular perturbation technique. The graphical results are discussed to see the effects of various physical parameters of interest. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21020