Numerical investigation of heat transfer and pressure drop characteristics of tube–fin heat exchangers in ice slurry HVAC system

This paper analyzes the heat transfer and pressure drop characteristics of a tube–fin heat exchanger in ice slurry HVAC system. Ice slurry is a suspension of crystallized water based - ice solution with a freezing point depressant like ethylene glycol. The ice- slurry is pumpable, hence it is also called pumpable ice. The composition of ice slurry considered for analysis is 14% ice fraction, 16% ethylene glycol, and 70% water by volume. It is deduced that the ice slurry HVAC system results in 7.4% increase in temperature drop over the conventional chilled water system The latent heat absorbed by ice slurry on melting makes it an attractive choice for achieving high degree of cooling. The numerical analysis was conducted by simulating the ice slurry tube flow region and air flow region of tube–fin heat exchanger in the air-handling unit of HVAC system. For the simulation six different louver patterns with 10 to 55 louver angle were considered. The design of the tube–fin heat exchanger for optimal heat transfer and pressure drop characteristics was also determined with the optimization parameter like louver angle, fin pitch, and ice slurry flow velocity.     

Analysis of the process characteristics of an absorption heat transformer with compact heat exchangers and the mixture TFE–E181

In the project described in this paper an experimental rig for a one-stage absorption heat transformer was designed and constructed. One aim of the project was to reduce the investment costs for the apparatus. This incorporates new and less expensive compact brazed plate heat exchangers for generator, evaporator, condenser and solution heat exchanger. The absorber was designed as a helical coil pipe absorber, where the weak solution trickles down as a falling film outside of the coil. The tests of the equipment involved measurements using a mixture of trifluorethanol (TFE) and tetraethyleneglycoldimethylether (E181). The process characteristics were investigated for different temperatures of the rich solution leaving the absorber. Experimental results are presented and compared with the results of a computer simulation model. Additionally the model was used to compare the COP of the heat transformation process with the mixtures lithium bromide–water (LiBr–H₂O) and ammonia–water (NH₃–H₂O). Furthermore, the overall heat and mass transfer coefficients for the plate heat exchangers and the falling film absorber were evaluated and compared with those of shell and tube heat exchangers.     

A new approach for the prediction of the heat transfer rate of the wire-on-tube type heat exchanger––use of an artificial neural network model

This study presents an application of artificial neural networks (ANNs) to predict the heat transfer rate of the wire-on-tube type heat exchanger. A back propagation algorithm, the most common learning method for ANNs, is used in the training and testing of the network. To solve this algorithm, a computer program was developed by using C++ programming language. The consistence between experimental and ANNs approach results was achieved by a mean absolute relative error <3%. It is suggested that the ANNs model is an easy modeling tool for heat engineers to obtain a quick preliminary assessment of heat transfer rate in response to the engineering modifications to the exchanger.     

Fluid flow and heat transfer characteristics of low temperature two-phase micro-channel heat sinks – Part 2. Subcooled boiling pressure drop and heat transfer

This second part of a two-part study explores the performance of a new cooling scheme in which the primary working fluid flowing through a micro-channel heat sink is indirectly cooled by a refrigeration cooling system. The objective of this part of study is to explore the pressure drop and heat transfer characteristics of the heat sink. During single-phase cooling, pressure drop decreased with increasing heat flux because of decreased liquid viscosity. However, pressure drop began increasing with increasing heat flux following bubble departure. These opposite trends produced a minimum in the variation of pressure drop with heat flux. Increasing liquid subcooling decreased two-phase pressure drop because of decreased void fraction caused by strong condensation at bubble interfaces as well as decreased likelihood of bubble coalescence. It is shown macro-channel subcooled boiling pressure drop and heat transfer correlations are unsuitable for micro-channel flows. However, two new modified correlations produced good predictions of the present heat transfer data.     

A new frontier of nanofluid research – Application of nanofluids in heat pipes

Nanofluid is a new kind of working fluid with special properties to enhance the heat transfer of heat pipes. This paper reviews and summarizes the research done on heat pipes using nanofluids as working fluids in recent years. The effect of characteristics and mass concentrations of nanoparticles on the thermal performance in various kinds of heat pipes with different base fluids under various operating conditions have been discussed. The mechanism of enhancement or degradation of heat transfer utilizing nanofluids in the investigated heat pipes has been explained. The paper discusses the relative reduction of the total heat resistance for various heat pipes with nanofluids in comparison with the existing ones and also presents a perspective on possible future research applications.     

Viscous Joule heating MHD–conjugate heat transfer for a vertical flat plate in the presence of heat generation

The problem of steady conjugate heat transfer through an electrically-conducting fluid for a vertical flat plate in the presence of transverse uniform magnetic field taking into account the effects of viscous dissipation, Joule heating, and heat generation is formulated. The general governing equations which include such effects are made dimensionless by means of an apposite transformation. The ultimate resulting equations obtained by introducing the stream function with the similarity variable are solved numerically using the implicit finite difference method for the boundary conditions based on conjugate heat transfer process. A representative set of numerical results for the velocity and temperature profiles, the skin friction coefficients as well as the rate of heat transfer coefficient and the surface temperature distribution are presented graphically and discussed. A comprehensive parametric study is carried out to show the effects of the magnetic parameter, viscous dissipation parameter, Joule heating parameter, conjugate conduction parameter, heat generation parameter and the Prandtl number on the obtained solutions.     

Boiling heat transfer and critical heat flux of ethanol–water mixtures flowing through a diverging microchannel with artificial cavities

This paper presents an experimental study on the convective boiling heat transfer and the critical heat flux (CHF) of ethanol–water mixtures in a diverging microchannel with artificial cavities. The results show that the boiling heat transfer and the CHF are significantly influenced by the molar fraction (x_m) as well as the mass flux. For the single-phase convection region except for the region near the onset of nucleate boiling with temperature overshoot, the single-phase heat transfer coefficient is independent of the wall superheat and increases with a decrease in the molar fraction. After boiling incipience, the two-phase heat transfer coefficient is much higher than that of single-phase convection. The two-phase heat transfer coefficient shows a maximum in the region of bubbly-elongated slug flow and deceases with a further increase in the wall superheat until approaching a condition of CHF, indicating that the heat transfer is mainly dominated by convective boiling. A flow-pattern-based empirical correlation for the two-phase heat transfer coefficient of the flow boiling of ethanol–water mixtures is developed. The overall mean absolute error of the proposed correlation is 15.5%, and more than 82.5% of the experimental data were predicted within a ±25% error band. The CHF increases from x_m = 0–0.1, and then decreases rapidly from x_m = 0.1–1 at a given mass flux of 175 kg/m² s. The maximum CHF is reached at x_m = 0.1 due to the Marangoni effect, indicating that small additions of ethanol into water could significantly increase the CHF. On the other hand, the CHF increases with increasing the mass flux at a given molar fraction of 0.1. Moreover, the experimental CHF results are compared with existing CHF correlations of flow boiling of the mixtures in a microchannel.     

Synthesis of heat exchanger networks featuring a minimum number of constrained-size shells of 1–2 type

In chemical plants requiring a high number of heat exchangers, standard sizes are established so that most of the services can be satisfied through arrays of a limited number of different standard-type units. If such an industrial practice is not taken into account during the heat recovery network synthesis task, the optimality of the proposed design could be doubtful. This paper addresses the HENS problem allocating multiple constrained-size shells rather than a single one to accomplish a heat match. Two cases are considered: (a) pure countercurrent exchangers and (b) shell-and-tube exchangers of 1–2 type. The neighbor-based HENS framework of Galli and Cerdá (1998) has been generalized in order to adopt a more realistic fixed-cost target, i.e. the overall number of constrained-size shells. Therefore, new 0–1 variables have been defined to stand for the additional shells needed to get both, a shell size below the specified upper bound, and simultaneously, an F_T correction factor above the threshold value everywhere. The resulting MILP problem formulation is now able to find network structures reaching the heat recovery target, under some structural constraints on the network design specified by the user, at near-minimum capital cost. The proposed algorithmic approach has been successfully applied to the solution of a couple of example problems and produced significant capital cost savings compared with prior HENS techniques.     

Heat transfer of gas–liquid mixture in micro-channel heat sink

The main objective of the present investigation is to study heat transfer in parallel micro-channels of 0.1 mm in size. Comparison of the results of this study to the ones obtained for two-phase flow in “conventional” size channels provides information on the complex phenomena associated with heat transfer in micro-channel heat sinks. Two-phase flow in parallel micro-channels, feeding from a common manifold shows that different flow patterns occur simultaneously in the different micro-channels: liquid alone (or single-phase flow), bubbly flow, slug flow, and annular flow (gas core with a thin liquid film, and a gas core with a thick liquid film). Although the gas core may occupy almost the entire cross-section of the triangular channel, making the side walls partially dry, the liquid phase always remained continuous due to the liquid, which is drawn into the triangular corners by surface tension. With increasing superficial gas velocity, a gas core with a thin liquid film is observed. The visual observation showed that as the air velocity increased, the liquid droplets entrained in the gas core disappeared such that the flow became annular. The probability of appearance of different flow patterns should be taken into account for developing flow pattern maps. The dependence of the Nusselt number, on liquid and gas Reynolds numbers, based on liquid and gas superficial velocity, respectively, was determined in the range of Re_LS = 4–56 and Re_GS = 4.7–270. It was shown that an increase in the superficial liquid velocity involves an increase in heat transfer (Nu_L). This effect is reduced with increasing superficial gas velocity, in contrast to the results reported on two-phase heat transfer in “conventional size” channels.     

Thermodynamic analysis of a combined PV/T–fuel cell system for power,heat, fresh water and hydrogen production

A hybrid renewable energy system is proposed and analyzed for electricity, heated air, purified water and hydrogen production. Energy, exergy and economic analyses are performed to analyze and determine the performance of the system under different operating conditions. The photovoltaic/thermal (PV/T) system produces heat and electricity for residential applications. Excess power is used to operate electrolyser which produces hydrogen to be fed directly to a fuel cell. Fuel cell is operated during high power demand, and it produces electricity, heat and water for residential applications. The water produced as a by-product by the fuel cell is used for drinking water supply. The parametric studies are conducted to determine the efficiencies of the system with and without fuel cell network for hot air, power and purified water. When fuel cell heat is used, the overall system efficiency increases to 5.65% for energy and 19.8% for exergy. Up to 80 L of drinkable water can be collected from the fuel cell when operated for extended periods. The present study confirms a significant economic gain when fuel cell heat and water are utilized as useful outputs.     

A new approach in evaluation of thermal contact conductance of tube–fin heat exchanger

This paper presents a new method in evaluating the thermal contact conductance (TCC) of tube–fin heat exchanger, which makes it possible to improve the tube–fin TCC performance at the stage of forming process design. Firstly, the tube–fin contact status is studied with a finite element (FE) model of tube expansion process. The simulation result shows that the tube–fin joining is far form full contact and a gap exists at the interface, which is confirmed by experimental observation. Distribution of the contact pressure along the tube–fin interface is obtained from the numerical results. Then, an experiment for the relationship between the contact pressure and the TCC is carried out. Combining the experiment result with the contact pressure distribution from the simulation, the tube–fin TCC can be evaluated. This evaluation results agree well with thermal measurement of the whole heat exchanger. Based on the method, effect of key factors of the expansion forming process, such as expanding ratio and die geometry, are optimized.     

Effects of heat absorption and thermal radiation on heat transfer in a fluid–particle flow past a surface in the presence of a gravity field

A continuum two-phase fluid–particle model accounting for fluid-phase heat generation or absorption and thermal radiation is developed and applied to the problem of heat transfer in a particulate suspension flow over a horizontal heated surface in the presence of a gravity field. Analytical solutions for the temperature distributions and the wall heat fluxes for both phases are obtained. Two cases of wall thermal conditions corresponding to stationary and periodic temperature distributions are considered. Numerical evaluations of the analytical solutions are performed and the results are reported graphically to elucidate special features of the solutions. The effects of heat absorption and thermal radiation are illustrated through representative results for the temperature distributions and heat fluxes of both phases for various fluid–particle suspensions. It is found that heat absorption increases the total heat transfer rate for various particulate volume fraction levels while thermal radiation decreases it.     

Can segmented flow enhance heat transfer in microchannel heat sinks?

Liquid cooling is an efficient way to remove heat fluxes with magnitudes up to 10,000 W/cm². One limitation of current single-phase microchannel heat sinks is the relatively low Nusselt number, due to laminar flow. In this work, we experimentally investigate how to enhance the Nusselt number with the introduction of segmented flow. The segmented flow pattern was created by the periodic injection of air bubbles through a T-junction into water-filled channels. We designed a polycarbonate heat sink consisting of an array of seven parallel microchannels each with a square cross-section 500 μm wide. We show that segmented flow increases the Nusselt number of laminar flow by more than 100%, provided the mass velocity of the liquid is within the range 330–2000 kg/m² s.     

Fluid flow and heat transfer in plasma reactors—I. Calculation of velocities,temperature profiles and mixing

A mathematical representation has been developed to describe the velocity field and the associated temperature and concentration fields in a plasma jet system, which involves the injection of additional gas streams. In the statement of the problem, allowance was made for the swirl of the plasma jet, and one important objective of the work was to explore the effect of this swirl on the principal process variables. It was found that swirl plays an important role in providing mixing between the plasma jet and a reactant or diluent gas stream introduced through an annular port. It was shown, furthermore, that the model may be used for representing the quenching of the system by an axi-symmetrically introduced gas stream, having a direction perpendicular to the axis of the jet.     

Distribution of air–water annular flow in a header of a parallel flow heat exchanger

The air and water flow distribution are experimentally studied for a heat exchanger composed of round headers and 10 flat tubes. The effects of tube protrusion depth as well as header mass flux, and quality are investigated, and the results are compared with previous 30 channel data. The flow at the header inlet is annular. For the downward flow configuration, water flow distribution is significantly affected by tube protrusion depth. For flush-mounted geometry, significant portion of water flows through frontal part of the header. As the protrusion depth increases, more water is forced to rear part of the header. The effect of header mass flux or quality is qualitatively the same as that of the protrusion depth. For the upward flow configuration, however, significant portion of water flows through rear part of the header. The effect of protrusion depth is the same as that of the downward flow. However, the effect of header mass flux or quality is opposite to the downward flow case. Compared with the previous 30 channel configuration, the present 10 channel configuration yields better flow distribution. Possible explanation is provided from flow visualization results.