Studies on pumping power in terms of pressure drop and heat transfer characteristics of compact plate-fin heat exchangers—A review

Renewable energy sources like solar energy, wind energy, etc. are profusely available without any limitation. Heat exchanger is a device to transfer the energy from one fluid to other fluid for many applications in buildings, industries and automotives. The optimum design of heat exchanger for minimum pumping power (i.e., minimum pressure drop) and efficient heat transfer is a great challenge in terms of energy savings point of view. This review focuses on the research and developments of compact offset and wavy plate-fin heat exchangers. The review is summarized under three major sections. They are offset fin characteristics, wavy fin characteristics and non-uniformity of the inlet fluid flow. The various research aspects relating to internal single phase flow studied in offset and wavy fins by the researchers are compared and summarized. Further, the works done on the non-uniformity of this fluid flow at the inlet of the compact heat exchangers are addressed and the methods available to minimize these effects are compared.     

Optimization of forced convection heat sinks with pumping power requirements

In this paper we address the optimization of a heat sink formed by parallel circular or non-circular ducts in a finite volume. The flow is considered to be laminar and steady and the fluid properties are assumed to be constant. Results for optimum dimensionless thermal length, optimum hydraulic diameter and maximum heat transfer rate density are presented for five different duct shapes subjected to a fixed pumping power constraint. Simple equations for the calculation of these optimum values are presented, and the influence of the local pressure drops at the inlet and outlet plenums, and of Prandtl number is discussed. The optimization results are then extended for the case of pumping power minimization at fixed heat transfer density.     

Experimental thermal–hydraulic evaluation of constructal microfluidic structures under fully constrained conditions

An experimental investigation was conducted to study the relative hydrodynamic and thermal performance of microfluidic, constructal-based, self-similar bifurcated flow channel arrangements with branching angles of 90°. The complexity of the microchannel arrangement was varied from zero to three bifurcation levels while the heat transfer area was held constant for all complexity levels. Constraining the area facilitates comparison of the thermal performance of test sections of different complexities. Each of the channel arrangements considered was incorporated into an independent, modular test section, which had overall dimensions of 10 mm by 10 mm. Using soft lithography and other standard microfabrication techniques, each test section was fabricated and assembled from a silicon heat transfer layer and two polydimethylsiloxane (PDMS) layers which were stacked and bonded to form a monolithic test section. For the testing, an experimental apparatus was designed that allowed for experiments to be run at fixed pressure drops. Experiments were performed for single fixed inlet fluid and heater temperatures and at various pressure drops. The results, which are reported in terms of mass flow rate, heat transfer rate, pumping power, and overall test section coefficient of performance (COP), indicate that complexity has a strong effect on both the pressure drop and heat transfer. When the pumping power required to produce a given heat transfer rate is taken into account, the results suggest that higher complexity arrangements can be beneficial under certain conditions, as theoretically shown in the literature. This conclusion is also confirmed by the trends observed in the COP.     

Entropy generation and pumping power in a turbulent fluid flow through a smooth pipe subjected to constant heat flux

In a heat exchange process, heat transfer and pumping power requirements are the two main considerations. Efforts made to increase heat transfer in a fluid flow usually cause increase in the pumping power requirement. In an effort to avoid inefficient utilization of energy through excessive entropy generation, a thermodynamic analysis of turbulent fluid flow through a smooth duct subjected to constant heat flux has been made in this study. The temperature dependence of the viscosity was taken into consideration in determining the heat transfer coefficient and friction factor. It was shown that the viscosity variation has a considerable effect on both the entropy generation and the pumping power. Pumping power to heat transfer ratio and the entropy generation per unit heat transfer can become very large especially for low heat flux conditions.     

Vascularization with line-to-line trees in counterflow heat exchange

Here we report the heat and fluid flow characteristics of counterflow heat exchangers with tree-shaped line-to-line flow channels. The flow structures of the hot and cold sides are sequences of point-to-line trees that alternate with upside-down trees. The paper shows under what conditions the tree vascularization offers greater heat flow access than corresponding conventional designs with parallel single-scale channels. The analytical part is based on assuming fully developed laminar flow in every channel and negligible longitudinal conduction in the solid. The numerical part consists of simulations of three-dimensional convection coupled with conduction in the solid. It is shown that tree vascularization offers greater heat flow access (smaller global thermal resistance) than parallel channels when the number of pairing levels increases and the available pumping power or pressure drop is specified. When the solid thermal conductivity increases, the heat transfer effectiveness decreases because of the effect of longitudinal heat conduction. The nonuniformity in fluid outlet temperature becomes more pronounced when the number of pairing levels increases and the pumping power (or pressure drop number) increases. The nonuniformity in outlet fluid temperature decreases when the solid thermal conductivity increases.     

Efficiency of optimized bifurcating tree-like and parallel microchannel networks in the cooling of electronics

A bifurcating tree-like network consists of a single inlet channel, which bifurcates over several levels to uniformly distributed microchannels that are vertically connected to a second network for fluid return. Here we introduce a one-dimensional model that considers convective heat transfer from the solid into the liquid as well as entrance and mixing effects. The performance of the bifurcating network is compared with that of a parallel microchannel cold plate branching from a single tapered manifold channel in terms of a constant volume flow rate, pressure gradient, and required pumping power. We optimized both networks independently with regard to global boundary conditions for cooling microprocessors and found a significantly superior performance for the parallel channel cooler. For a constant flow rate, the parallel channel network achieves a more than fivefold higher performance coefficient than the bifurcating tree-like network, while almost four times more heat can be removed for a constant pressure gradient across the networks.     

New challenge in advanced thermal energy transportation using functionally thermal fluids

The present review article presents the current status of some researches on thermal energy transportation using functionally thermal fluid, which is a mixture of heat transfer medium like water and other material with or without phase change like a paraffin wax as a latent heat storage material. This functionally thermal fluid offers attractive opportunities for thermal energy transportation and heat transfer enhancement of heat exchanger. This article describes classification and characteristics of functionally thermal fluids and their application. Referring to functionally thermal fluid for the usage of sensible heat, some visco-elastic fluids for flow drag reduction in a thermal energy transport system such as aqueous polymer solution and surfactant solution are mentioned. On the other hand, this article describes heat transfer and hydrodynamic characteristics of some phase change slurries like ice slurry, phase change microemulsion slurry, phase change microencapsule slurry, clathrate slurry and shape-stabilized paraffin and polyethylene pellets as functionally thermal fluids using latent heat between solid and liquid phases. Finally, it leads to the conclusion that some functionally thermal fluids are very useful for the advanced thermal energy transportation and heat exchanger systems.     

Theoretical and computational analyses of LNG evaporator

Theoretical and numerical analysis on the fluid flow and heat transfer inside a LNG evaporator is conducted in this work.Methane is used instead of LNG as the operating fluid.This is because;methane constitutes over 80％ of natural gas.The analytical calculations are performed using simple mass and energy balance equations.The analytical calculations are made to assess the pressure and temperature variations in the steam tube.Multiphase numerical simulations are performed by solving the governing equations (basic flow equations of continuity,momentum and energy equations) in a portion of the evaporator domain consisting of a single steam pipe.The flow equations are solved along with equations of species transport.Multiphase modeling is incorporated using VOF method.Liquid methane is the primary phase.It vaporizes into the secondary phase gaseous methane.Steam is another secondary phase which flows through the heating coils.Turbulence is modeled by a two equation turbulence model.Both the theoretical and numerical predictions are seen to match well with each other.Further parametric studies are planned based on the current research.     

Experimental study on liquid/solid phase change for cold energy storage of Liquefied Natural Gas (LNG) refrigerated vehicle

The present paper addresses an experimental investigation of the cold storage with liquid/solid phase change of water based on the cold energy recovery of Liquefied Natural Gas (LNG) refrigerated vehicles. Water as phase change material (PCM) was solidified outside the heat transfer tubes that were internally cooled by cryogenic nitrogen gas substituting cryogenic natural gas. The ice layer profiles were recorded in different cross-sections observed by digital cameras. The temperatures of cryogenic gas, tube wall and bulk region were measured by embedded thermocouples continuously. The results of the smooth tube experiments and the thermal resistance analysis prove that the main thermal resistance occurs in the gaseous heat transfer fluid (HTF) inner the tube. The enhancement of the inner heat transfer is achieved by adding wave-like internal fins. Besides, the results show that the ice layer not only increases in radial direction but also propagates in axial direction. It distributes in parabolic shape along the tube length due to the parabolic axial distribution of the tube wall temperatures. This investigation provides valuable references for the design and optimization of the cold energy storage unit of LNG refrigerated vehicles and for the numerical study on the unsteady two-dimensional conjugated heat transfer with phase change.     

Experimental and numerical study on charging processes of an ice‐on‐coil thermal energy storage system

In this study, an external melt ice‐on‐coil thermal storage was studied and tested over various inlet conditions of secondary fluid—glycol solution—flow rate and temperature in charging process. Experiments were conducted to investigate the effect of inlet conditions of secondary fluid and validate the numerical model predictions on ice‐on‐coil thermal energy storage system. The total thermal storage energy and the heat transfer rate in the system were investigated in the range of 10 l min ^?1?V??60 l min ^?1. A new numerical model based on temperature transforming method for phase change material (PCM) described by Faghri was developed to solve the problem of the system consisting of governing equations for the heat transfer fluid, pipe wall and PCM. Numerical simulations were performed to investigate the effect of working conditions of secondary fluid and these were compared with the experimental results. The numerical results verified with experimental investigation show that the stored energy rises with increasing flow rate a decreasing tendency. It is also observed that the inlet temperature of the fluid has more influence on energy storage quantity than flow rate. Copyright © 2006 John Wiley & Sons, Ltd.     

Three-Dimensional Numerical Study of Fluid and Heat Transfer Characteristics of Dimpled Fin Surfaces

In the present study, a code based on the nonorthogonal curvilinear coordinates is developed with a collocated grid system generated by the two-boundary method. After validation of the code, it is used to compare simulated results for a fin-and-tube surface with coupled and decoupled solution methods. The results of the coupled method are more agreeable with the test data. Simulation for dimpled and reference plain plate fin-and-tube surfaces are then conducted by the coupled method within a range of inlet velocity from 1.0 m/s to 5 m/s. Results show that at identical pumping power the dimpled fin can enhance heat transfer by 13.8–30.3%. The results show that relative to the reference plain plate fin-and-tube surface, heat transfer rates and pressure drops of the dimpled fin increase by 13.8%–30.3% and 31.6%–56.5% for identical flow rate constraint. For identical pumping power constraint and identical pressure drop constraint, the heat transfer rates increase by 11.0%–25.3% and 9.2%–22.0%, respectively. By analyzing the predicted flow and temperature fields it is found that the dimples in the fin surface can improve the synergy between velocity and fluid temperature gradient.     

NUMERICAL STUDY ON COOLING PHENOMENON OF WATER WITH SUPERCOOLED REGION IN A HORIZONTAL CIRCULAR CYLINDER

Ice formation in a horizontal circular has been studied numerically. From the numerical analysis results, it was found that there were three types of freezing patterns and that the freezing phenomenon was affected largely by density inversion and cooling rate. The type of freezing pattern largely depends on the secondary flow, which is generated by density inversion. When supercooling energy is released before the development of the secondary flow, an annular ice layer grows. If the energy is released when the secondary flow is considerably developed and the supercooled region is removed to the upper half part of the cylinder, an asymmetric ice layer grows. If the energy is released after perfect development of the secondary flow, instantaneous dendritic ice formation over the full region occurs. Furthermore, the secondary flow was found to have an effect on heat transfer characteristics. The heat transfer rate becomes small at the instant when the secondary flow is generated, but it becomes large with the development of the secondary flow. It is concluded that for the facilitation of heat transfer it is desirable to keep water, in its liquid phase until the secondary flow is perfectly developed. This study gives an instruction on the performance improvement of a capsule-type ice storage tank.     

An analysis of the heat transfer rate and efficiency of TE (thermoelectric) cooling systems

The heat transfer rate and efficiency of TE (thermoelectric) cooling systems were investigated. The emphasis of the present study is focused on the use of large-scale TE refrigerators for air conditioning applications. A one-dimensional heat transfer analysis was performed to determine the cooling power and electricity consumption of the TE elements. The constant-property results are in good agreement with the variable-property solutions for TE materials and temperatures typical for air conditioning applications. A heat transfer analysis was also carried out for TE refrigerators equipped with a heat exchanger. Both parallel- and counter-flow heat exchangers were considered. Fluid temperature variations of these two flow arrangements were found to be quite different, but the efficiencies and cold fluid exit temperatures differed only slightly when a uniform current was used for all TE elements. If the length of the heat exchanger exceeds an optimal value, the cold fluid temperature begins to rise and the efficiency drops for both parallel- and counter-flow arrangements. The second law of thermodynamics was applied to the optimization of TE refrigerators operating between two constant-temperature reservoirs and between two flowing fluids. It was found that if a TE cooling system incorporates a heat exchanger, a nonuniform current distribution should be used to achieve the maximum efficiency and the lowest cold fluid temperature. The optimization results for TE refrigerators operating between two constant-temperature reservoirs are not applicable to TE cooling systems between two flowing fluids. The most energy-efficient current distribution for the parallel-flow arrangement is the one which increase in the direction of the cold fluid.     

Performance of ice slurry as cooling fluid for an indoor ice rink

A slightly modified version of a previously published model calculating transient heat transfer under the ice of an indoor ice rink is used to evaluate the performance of two cooling fluids, a brine with 20 % calcium chloride (base case) and a calcium chloride ice slurry. Simulations are conducted for a typical meteorological year for Montreal, Canada and take into account heat entering the ice from above as well as surfacing operations and electrical underground heating used to avoid freezing which can damage the concrete slab. The results show that, for the same flow rate (28.5 l/s) and inlet temperature (?9 °C) of the cooling fluid, the ice slurry generates better ice quality (ice surface temperature is more uniform spacewise and less variable with time) but requires more pumping power. Parametric results obtained by decreasing the flow rate or by increasing the inlet temperature of the ice slurry indicate that it is possible to choose either of these operating parameters so that the resulting ice quality is better and the pumping power is lower than for the base case.     

Energetic and Exergetic Performances of Plate Heat Exchanger Using Brine-Based Hybrid Nanofluid for Milk Chilling Application

Abstract

Theoretical study on the energetic and exergetic performances of a counter-flow corrugated plate heat exchanger using hybrid nanofluids for the milk chilling application has been done in the present investigation. Magnesia-silver and Alumina-silver nanoparticles have been dispersed in the ethylene glycol–water mixture and propylene glycol–water mixture (20:80 brine solutions) with different particle volume concentration separately. Effect of particle volume concentration and flow rate of the hybrid nanofluid on the heat transfer rate, convective, and overall heat transfer coefficients, mass flow rate of milk, pressure drop, pumping power, entropy generation rate, second law efficiency, irreversibility, irreversibility distribution ratio, non-dimensional exergy (NDE) destruction, and performance index have been studied. It has been observed that heat transfer rate, convective and overall heat transfer coefficients, pressure drop, pumping power, irreversibility, entropy generation rate, second law efficiency, and milk flow rate increase; while NDE destruction, performance index, and irreversibility distribution ratio decrease with the hybrid nanofluid flow rate and the volume concentration of the nanofluid. Within studied ranges, the hybrid nanofluid yields the maximum improvement of heat transfer rate and convective heat transfer coefficient of about 1.6% and 9.4%, respectively, compared to base fluid. It has also been found that silver?+?alumina shows slightly better performance improvement and hence hybrid nanofluid is recommended as a suitable alternative for the milk chilling units.     

Experimental investigation of thermoelectric power generation versus coolant pumping power in a microchannel heat sink

The coolant heat sinks in thermoelectric generators (TEG) play an important role in order to power generation in the energy systems. This paper explores the effective pumping power required for the TEGs cooling at five temperature difference of the hot and cold sides of the TEG. In addition, the temperature distribution and the pressure drop in sample microchannels are considered at four sample coolant flow rates. The heat sink contains twenty plate-fin microchannels with hydraulic diameter equal to 0.93 mm. The experimental results show that there is a unique flow rate that gives maximum net-power in the system at the each temperature difference.     

Application of nanofluids in heating buildings and reducing pollution

This paper presents nanofluid convective heat transfer and viscosity measurements, and evaluates how they perform heating buildings in cold regions. Nanofluids contain suspended metallic nanoparticles, which increases the thermal conductivity of the base fluid by a substantial amount. The heat transfer coefficient of nanofluids increases with volume concentration. To determine how nanofluid heat transfer characteristics enhance as volume concentration is increased; experiments were performed on copper oxide, aluminum oxide and silicon dioxide nanofluids, each in an ethylene glycol and water mixture. Calculations were performed for conventional finned-tube heat exchangers used in buildings in cold regions. The analysis shows that using nanofluids in heat exchangers could reduce volumetric and mass flow rates, and result in an overall pumping power savings. Nanofluids necessitate smaller heating systems, which are capable of delivering the same amount of thermal energy as larger heating systems using base fluids, but are less expensive; this lowers the initial equipment cost excluding nanofluid cost. This will also reduce environmental pollutants because smaller heating units use less power, and the heat transfer unit has less liquid and material waste to discard at the end of its life cycle.     

Effect of oscillatory frequency on heat transfer in metal foam heat sinks of various pore densities

In this paper, an experimental investigation was performed to study the heat transfer performance of metal foam heat sinks of different pore densities subjected to oscillating flow under various oscillatory frequencies. The variations of pressure drop and flow velocity along the kinetic Reynolds number of oscillating flow through aluminum foams were compared. The measured pressure drops, velocities and surface temperatures of oscillating flow through aluminum 10, 20 and 40 PPI foams were presented in detail. The calculated cycle-averaged local temperature and Nusselt number for different kinetic Reynolds numbers were analyzed and compared with finned heat sinks. The results of length-averaged Nusselt number for both oscillating and steady flows indicate that higher heat transfer rates can be obtained in metal foams subjected to oscillating flow. For the purpose of designing a novel heat sink using metal foam, the characteristics of the pumping power of the cooling system for aluminum foam with different pore densities were also analyzed.     

相变微胶囊悬浮液强制对流换热实验研究

提出了一种可以同时作为储能介质与传热流体的新型相变微胶囊悬浮液(MPCS),设计和搭建试验台,分别在层流和湍流条件下在等热流密度的光滑圆管中对MPCS进行了强制对流换热实验,研究了悬浮液浓度、流量、泵送功率和加热速率对其流动及传热特性的影响。结果表明:对于质量分数为5%的MPCS,当微胶囊中相变材料分别处于固体、固体-液体和液体状态时,对应的努塞尔数平均增大了23.9%、20.5%和9.1%;与纯水相比,MPCS作为在热力系统应用的传热流体可以实现强化传热,但是需要在传热实验中控制好相变过程才能使MPCS的传热性能优于水。     

Validation of the use of heat transfer models in liquid/solid fluidized beds for ice slurry generation

Heat transfer coefficients in a liquid/solid fluidized bed heat exchanger are investigated for application in ice slurry generators. A range of temperature driving forces is determined in which ice slurry generation is stable. In this range ice crystal formation or growth does not affect heat transfer coefficients. A model is proposed that accurately predicts heat transfer coefficients in the fluidized bed ice slurry generator. Due to lower temperatures and higher viscosity in ice slurry generation, heat transfer coefficients measured are lower than predicted with heat transfer correlations specific for liquid/solid fluidized bed heat exchangers. Heat transfer coefficients measured are however significantly higher than for single phase fluid flow.