A mechanistic heat transfer correlation is proposed to estimate heat transfer coefficient for non-boiling two-phase flow in horizontal, slightly inclined, and vertical pipes using the analogy between friction factor and heat transfer. Local heat transfer coefficients, pressure drops and flow parameters were measured for air-water flow in a 27.9 mm stainless steel pipe. The heat transfer and pressure drop data were collected by carefully coordinating the gas and liquid superficial Reynolds numbers. The proposed mechanistic correlation is validated by using experimentally measured heat transfer data. Evaluation of the mechanistic correlation with the measured heat transfer data indicated that the analogy between friction factor and heat transfer can be used with reasonable accuracy for heat transfer predictions in non-boiling two-phase pipe flow. Comparison with experimental results showed that the bulk of the data points were predicted within ±30% by the mechanistic model.

This study investigates pressure drop associated with water liquid single-phase flow across an array of staggered micro-pin-fins having circular cross-section. The micro-pin-fins are micro-end milled out of oxygen free copper and have the following dimensions: 180 micron diameter and 683 micron height. The longitudinal pitch and transverse pitch are equal to 400 microns. Seven water inlet temperatures from 22 to 80 °C, and seventeen maximum mass velocities for each inlet temperature, ranging from 159 to 1475 kg/m 2 s, were tested. The test module was well insulated to maintain adiabatic conditions. The experimental results were compared to those from a micro-pin-fin array having similar size and geometrical arrangement but a square cross-section. The circular micro-pin-fins were seen to yield a significantly lower pressure drop than the square micro-pin-fins. The present experimental results were also compared with the predictions of several friction factor correlations as well as the results from a three-dimensional numerical analysis. Neither was able to accurately predict the experimental data.

The effects of pitch and depth on the condensation heat transfer of R-134a flowing inside corrugated tubes are experimentally investigated. The test section is a horizontal tube-in-tube heat exchanger. The refrigerant flows in the inner tube and the water flows in the annulus. The length of heat exchanger is 2 m. A smooth tube and corrugated tubes having inner diameters of 8.7 mm are used as an inner tube. The corrugation pitches used in this study are 5.08, 6.35, and 8.46 mm. Similarly, the corrugation depths are 1, 1.25, and 1.5 mm. The effects of corrugation pitch and depth on tube wall temperature, heat transfer coefficient and frictional pressure drop are discussed. The results illustrate that the maximum heat transfer coefficient and frictional pressure drop obtained from the corrugated tube are up to 50% and 70% higher than those obtained from the smooth tube, respectively.

Microtextured superhydrophobic surfaces have become ubiquitous in a myriad of engineering applications. These surfaces have shown potential in friction reduction applications and could be poised to make a big impact in thermal management applications. For instance higher heat transfer rate with less pumping power might be achievable through the aid of superhydrophobic surfaces. However, past and current research on superhydrophobic surface has focused mainly on modifying either the chemical component or the roughness factors of such surfaces. The purpose of this paper is to account for the thermal effects of the heated fluid flowing in superhydrophobic microfluidic channels. Herein we characterize the wetting behavior as a function of temperature of microtextured superhydrophobic surfaces, for both active and passive thermal management applications. A series of PDMS microtextured samples were fabricated using micromachining and soft lithography techniques. Flow measurements were performed using the superhydrophobic microfluidic channel. The channel surface roughness was large enough to induce the Cassie-Baxter state, a phenomenon in which a liquid rests on top of a textured surface with a gas layer trapped underneath the liquid layer. This gas layer induces a two-phase flow, and friction reduction can be achieved for the liquid channel flow. With this channel, flow rates were measured by varying the equilibrium temperature of the substrate. The temperature in the constant pressure source was controlled by circulating the water through a water-bath. As the heating reached a certain threshold the curvature of the liquid-gas interface was reversed and dewetting of the penetrated liquid layer was observed. This result suggests that the Cassie state in fluid flow can be prolonged even under increased pressure drops by increasing the temperature in the gas layer.

Two design options of core distribution block (CDB) for a cooled-vessel design in the Very High Temperature Reactor (VHTR) were developed and the influence on the core hot spot was investigated by the commercial computational fluid dynamics (CFD) code, CFX-11. Isothermal CFD analyses were performed to estimate the coolant flow variation at the inlet of the coolant channel. The results predicted about 5% of the maximum velocity deviation when applying the core pressure drop of NHDD PMR200. A unit-cell CFD model was used to access the effect of the velocity deviation on the core hot spot. The unit-cell analyses were carried out for the velocity deviation of 0%, 5%, and 10%. Not only a constant power but also a local maximum power profile was considered. According to the results, the maximum fuel temperature was increased by about 30°C for the velocity deviation of 10% but still below the normal operation limit of 1250°C.

To increase heat transfer, internally micro-fin tubes are widely used in commercial HVAC applications such as flooded evaporators. It is commonly understood that the micro-fin enhances heat transfer but at the same time increases the pressure drop as well. In the previous studies, majority of the works were focused on the development of correlations in a particular flow regime, especially in the turbulent region. There are only a few works that fundamentally studied the continuous change in the characteristic behavior of heat transfer and pressure drop from laminar to transition and eventually the turbulent regions. Therefore, more in-depth study is necessary. In this study, heat transfer and pressure drop were measured simultaneously in a single test section fitted with 2 micro-fin tubes and compared with the data of a plain tube. From the results, the transition from laminar to turbulent was clearly established. The buoyancy effect is present in the laminar region. The transition from laminar to turbulent was found to be inlet dependent. It could be seen that the delay of transition was more obvious for smaller spiral angle while it was not as obvious when large spiral angle tube was used. Furthermore, it was observed that the larger spiral angle had an enhancement of the heat transfer in the upper transition to turbulent regions. Finally, the efficiency index (the ratio of the heat transfer and the friction factor of enhanced tube to those variables for the plain tube) was examined and it was found to have a value larger than one when Reynolds number is larger than 5000 regardless of the type of inlet configuration used. Therefore, the application of the micro-fin tubes used in this study is suitable when Reynolds number is larger than 5000.

A mathematical model of lime calcination process in normal shafts kiln has been developed to determine the heat and mass transfer between the gas and the solid. The model is one-dimensional and steady state. The transport of mass and energy of the gas and the solid is modeled by a system of ordinary differential equations. A shrinking core approach is employed for the mechanics and chemical reactions of the solid material. The model can be used to predict the temperature profiles of the particle bed, the gas phase along the length of kiln axis. The calcination behavior of the particle bed can be also investigated. The influences of operational parameters such as: energy input, the origin of feed limestone and the lime throughput on the kiln performance including pressure drop are considered. Additionally, the local heat loss through the kiln wall is studied. The results of this study are direct utility for optimization and design of large-scale technical shaft kilns.

A microstructure device for cooling of hot surfaces at liquid single phase laminar flow is presented. The initial design as well as the theoretical background is described in detail. It consists of numerous short micro channels acting as overflow structures and providing a relatively large hydraulic diameter, used in parallel between large inlet and outlet channels. The design was chosen to be scalable as well as appropriate for mass production in different materials. The fluid distribution was optimized as well as the dimensions of the overflow structures in terms of heat transfer, both by CFD simulations. Several devices were tested. They provide very high heat flux at reasonably low pressure drop. The temperature difference to achieve, heat flux and pressure drop can be adjusted easily by control of the applied mass flow. The design was tested as liquid-liquid heat exchanger in a simple lab-scale test facility. Moreover, using a copper electrically powered surface heat focus, some devices were tested as surface coolers.

Numerical analyses for pressure drop and heat transfer in the flow channels of a printed circuit heat exchanger have been performed numerically. Three-dimensional Reynolds-averaged Navier-Stokes equations have been solved in conjunction with the shear stress transport model as a turbulence closure. The numerical solutions are validated with the available experimental results of the reference shape. The effects of two design parameters, namely, the channel angle and the ellipse aspect ratio of the cold channel, on the heat transfer and the friction performance have been evaluated.

This paper presents an analytical investigation of the thermally developing and periodically fully-developed flow in a parallel-plate channel comprised of superhydrophobic walls. The superhydrophobic walls considered in this paper exhibit alternating micro-ribs and cavities positioned perpendicular to the flow direction and the transport scenario analyzed is that of constant wall heat flux through the rib surfaces with negligible thermal transport through the vapor cavity interface. Axial conduction is neglected in the analysis and the problem is one of Graetz flow with apparent slip-flow and periodicity of constant heating. Closed form solutions for the local Nusselt number and wall temperature are presented and are in the form of infinite series expansions. Previously it has been shown that significant reductions in the overall frictional pressure drop can be expected relative to the classical smooth channel laminar flow. The present results reveal that the overall thermal transport is markedly influenced by the relative cavity region (cavity fraction), the relative rib/cavity module width, and the flow Peclet number. The following conclusions can be made regarding thermal transport for a constant heat flux channel exhibiting the superhydrophobic surfaces considered: 1) Increases in the cavity fraction lead to decreases in the average Nusselt number; 2) Increasing the relative rib/cavity module length yields a decrease in the average Nusselt number; and 3) as the Peclet number increases the average Nusselt number increases. For all parameters explored, the limiting upper bound on the fully-developed average Nusselt number corresponds to the limiting case scenario of classical laminar flow through a smooth-walled channel with constant heat flux.

Heat transfer and pressure drop measurements for horizontal macro-tubes under uniform wall heat flux boundary condition have been conducted by various researchers in recent years. From their studies, it was shown that good agreements were observed in the laminar and turbulent regions. However, for the transition region, the heat transfer and pressure drop characteristics depended on various factors, such as inlet configuration, buoyancy effect, and surface roughness. In a recent study by Tam et al. (2010), they measured the heat transfer and pressure drop simultaneously for a horizontal macro-tube with and without internally micro-fins and concluded that under the heating condition, the transition Reynolds number range for heat transfer and pressure drop were completely different. The transition Reynolds number range was documented in their research in great detail. However, for horizontal micro-tubes, there is no information in the literature on the simultaneous behavior of the heat transfer and pressure drop, especially in the transition region. In order to fill in this gap, an experimental setup was built to measure the heat transfer and pressure drop simultaneously for a horizontal micro-tube under uniform wall heat flux boundary condition. Water was used as the test fluid and the test section was a stainless steel micro-tube with 1000μm diameter. For heat transfer, the results indicated that the micro-tube had an earlier start and end of transition compared to the macro-tube and, in the turbulent region, an increase in heat transfer due to the surface roughness was observed. For friction factor under isothermal condition, the micro-tube had a narrower transition range due to the roughness compared to the macro-tube. For friction factor under heating condition, the laminar data and the start of transition were different from the isothermal case, and the effect of heating was not seen on the end of transition.

The performance of two-phase flow through fractal-like heat sinks, subject to both geometrical and flow constraints was assessed. Constraints are crucial in order to satisfy physical requirements of a design. A one-dimensional model of two-phase flow through fractal-like branching microchannels was used to estimate pressure drop, wall temperature and critical heat flux. Water is employed as the working fluid. The exit pressure is varied between 6 kPa and 101.3 kPa (absolute) in order to achieve two-phase flow at temperatures lower than the maximum wall temperature constraint of 70°C. Preliminary results show that the benefit to cost ratio of two-phase flow is on the same order of magnitude as single-phase flow, both with a 70°C wall temperature constraint. Alternatively, a critical heat flux model is used to constrain the flow rate in order for the imposed heat flux to be 50% of the critical heat flux.

Experimental adiabatic two-phase pressure drops data for refrigerants R134a, R236fa and R245fa during flow boiling in small channels with internal diameters of 1.03, 2.20 and 3.04 mm are presented. The main purpose was to investigate the effects of channel confinement on adiabatic two-phase pressure drops. Thus, the two-phase pressure drop trends were systematically investigated over a wide range of test conditions for all three refrigerants and channel sizes. Statistical comparisons have also been made by comparing the experimental pressure drop data database with various macroscale and microscale prediction methods from the literature. The comparison showed relatively moderate accuracy for three prediction methods developed for macroscale flows, i.e. Baroczy and Chisholm, Friedel and the homogeneous model with the Cicchitti et al. viscosity relation. As for microscale prediction methods, the Cioncolini et al. annular flow model worked best with 68.5% of the data within ± 30%, followed by the Sun and Mishima and the Zhang et al. methods. Combining this database with the LTCM lab’s earlier database for 0.509 and 0.790 mm channels, there appears to be no evidence of a macro-to-microscale transition, at least with respect to two-phase pressure drops.

It can be difficult to burn relatively cheap, poor quality, unprepared biomass materials in industrial heating processes because of their variable composition, relatively low calorific values and high moisture contents. Consequently the stability and efficiency of the combustion process can be adversely affected unless they are co-fired with a hydrocarbon support fuel. There is a lack of information on the “optimum” conditions for co-firing of coal and high moisture biomass as well as on the proportions of support fuel which should be used. This paper is therefore concerned with the pilot scale (<25 kW thermal input) fluidised bed combustion of blends of coal with pressed sugar beet pulp, a solid biomass with an average moisture content of 71%. The experimental work was undertaken in collaboration with British Sugar plc who operate a coal-fired 40 MW thermal capacity fluidised bed producing hot combustion gases for subsequent drying applications. The project studied the combustion characteristics of different coal and pressed pulp blends over a wide range of operating conditions. It was found that stable combustion could only be maintained if the proportion of pulp by mass in the blended fuel was no greater than 50%. However evaporation of the moisture in the pressed pulp cools the bed so that the excess air which is necessary to maintain a specified bed temperature at a fixed thermal input can be reduced as the proportion of biomass in the blended fuel is increased. Therefore, with a 50/50 blend the bed can be operated with 20% less fluidising air and this will be beneficial for the output of the full scale plant since at present the flow rate of the air and hence the amount of coal which can be burnt is restricted by supply system pressure drop limitations. A further benefit of co-firing pressed pulp is that NOx emissions are reduced by about 25%. Agglomeration of the bed can be a problem when co-firing biomass because of the formation of “sticky” low melting point alkali metal silicate eutectics which result in subsequent adhesion of the ash and sand particles. Consequently longer term co-firing tests with a 50/50 blended fuel by mass were undertaken. Problems of bed agglomeration were not observed under these conditions with relatively low levels of alkali metals in the ash.

The two-phase friction factor of R134a in a copper smooth tube having an inner diameter of 8.1 mm and length of 0.5 m during downward condensation is investigated experimentally and numerically. The test runs were performed at average condensing temperatures of 40–50 °C. The mass fluxes were around 260, 340 and 456 kg m −2 s −1 and the heat fluxes were between 11.3 and 55.3 kW m −2 . Accuracy of the dataset was proven in many papers in the literature. The quality of the refrigerant in the test section is calculated considering the temperature and pressure obtained from the experiment. The pressure drop across the test section is directly measured by a differential pressure transducer. The equivalent Reynolds number is considered to be the significant variable for the analysis. A Genetic Algorithm (GA), is one of the most successful methods among evolution algorithms, is applied for the optimization of the relationship between the two-phase friction factor and Reynolds equivalent number model in this study. The annular flow condensation process in the vertical test tube is expressed using GA method successfully. Regression analysis was done including the Reynolds equivalent number and other measured values such as pressure drop and mass flux of R134a, and gave a convincing correlation based on 182 smooth tube data points for practical applications. The most suitable coefficients of the proposed correlations are depicted to be compatible with the experiment by way of the algorithm.

In this study the pressure drop and local heat transfer coefficient were investigated experimentally during the convective condensation of R-134a inside eight round (D = 0.8 mm) horizontal and parallel microchannels. The test conditions included mass velocity and pressure ranges of 57 to 125 kg.m −2 s −1 and 6.8 to 11.2 bar, respectively. The experimental results are compared with correlations and semi-empirical models described in the literature.

This study experimentally assesses single phase heat transfer characteristics of a shallow rectangular microchannel heat sink whose surface is enhanced with copper nanowires (CuNWs). The hydraulic diameter of the channel is 672 μm and the bottom wall is coated with Cu nanowires (CuNWs) of 200 nm in diameter and 50 μm in length. CuNWs are grown on the Cu heat sink by electrochemical synthesis technique which is inexpensive and readily scalable. The heat transfer and pressure drop results of CuNWs enhanced heat sink are compared with that of bare copper heat sink using deionized (DI) water as the working fluid at Reynolds Number (Re) ranging from 106–636. The experimental results indicate an enhancement in Nusselt Number (Nu) at all Re with a maximum enhancement of 24% at Re = 106. The enhanced thermal performance is attributed to two properties of Cu nanowire arrays — improvement in surface wettability characteristics and increased heat transfer surface area.

Entropy generation rate has been the attraction of research, since it provides information on the thermodynamic irreversibility associated with the thermal systems. The exergy distraction in the thermal system increases entropy generation rate while lowering the second law efficiency of the thermal system. The heat transferring devices, such as heat exchangers, operates better when temperature difference between the transferring device and the heat sink is maintained high. In addition, the use of porous material in these devices enhances the heat transfer rates due to the achievement of high heat transfer coefficients. However, the presence of the porous material also increases the pump power because of the high pressure drop in the flow system. This increases the operational costs. Consequently, entropy generation rate due to pressure drop needs to be minimized to reduce the cost; however, heat transfer rates from the thermal system needs to be enhanced to improve the thermal performance of the heat transferring device. Therefore, a balance between the entropy generation rates due to pressure drop and heat transfer needs to be attained to achieve optimum operating conditions of such devices. To investigate the optimum operating conditions, the forced convection problem about inclined surfaces (or wedges) in saturated porous medium is considered. The flow in the porous medium is described by the Darcy-Brinkman momentum equation. An exact analytical solution of the governing equations using Kummer function is developed for the velocity, temperature, Nusselt Number, and entropy generation rate for the case where the free stream velocity and wall temperature distribution of the inclined surface vary according to the same power function of distance x , along the plate. It is demonstrated that the entropy generation number is weakly dependent on the Brinkman-Darcy number for forced convection flow, which is particularly true near the wall region.

The plate-pin fin heat sink (PPFHS) is composed of a plate fin heat sink (PFHS) and some pin fins planted between the flow channels. In this paper, a numerical investigation was performed to compare the thermal and hydraulic performances of the PPFHSs and PFHS. PPFHSs with five forms of pin cross-section profiles (square, circular, elliptic, NACA 0050, and dropform) were numerically simulated. The influence of pin fin cross-section profile on the flow and heat transfer characteristics was presented by means of Nusselt number and pressure drop. It is found that the Nu number of a PPFHS is at least 35% higher than that of a PFHS used to construct the PPFHS at the same Reynolds number. Planting circular and square pins into the flow channel of heat sinks enhances the heat transfer at the expense of high pressure loss. Using the streamline shaped pins, not only the pressure drop of the compound heat sinks could be decreased considerably, the heat transfer enhancement also makes a step forward. The present numerical simulation provides original information of the influence of different pin-fin cross-section profiles on the thermal and hydraulic performance of the new type compound heat sink, which is helpful in the design of heat sinks.

A steady-state two dimensional numerical simulation was carried out to optimize the heat transfer rate density from cylinders under different conditions. The geometric design of the cylinders was varied in two ways. In the first case the cylinders are located on a plane where their leading edges are aligned, and in the second case the cylinders are aligned on a plane which passes through their respective centre-lines. The rotation of the cylinders is within the range of 0 ≤ ω˜ ≤ 1, and the dimensionless pressure drop number, Be, which drives the flow is in the range of 10 ≤ Be ≤ 10 4 . The continuity, momentum and energy equations describing the flow of the coolant, across the cylinders in the computational domain are performed using a computational fluid dynamics code, the results obtained were validated by comparing it with past results in the open literature for stationary cylinders. The effects of the various parameters (dimensionless pressure drop number, rotation) on the maximum heat transfer rate density from the cylinders in terms of augmentation and the suppression were analysed and reported.