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Copyright © 2023 The authors. This study proposed newly designed inlet manifolds to manage non-uniformity in parallel multi microchannel heat sink by introducing edges with a curved shape were introduced in order to reduce flow-recirculation at the sharp edges. This resulted in a better flow distribution in the parallel channels. A comprehensible numerical study has been performed using ANSYS-Fluent and a three-dimensional computational domain, incorporating the effect of conjugated heat transfer, was employed in this study. R134a was used as the working fluid and copper was selected as the heat sink material. The dimensionless channel flow ratio and flow maldistribution factor were introduced to quantify the flow distribution inside individual channels and the uniformity of this flow distribution. A uniform flow distribution is achieved when the maldistribution factor value approaches 0. Engineering & Physical Sciences Research Council EP/T033045/1 BOiliNg flows in SmAll and microchannels (BONSAI), The authors would like to express their appreciation to the Ministry of Higher Education Malaysia and Research Management Centre Universiti Malaysia Perlis (project number FRGS/1/2018/TK07/UNIMAP/02/2) for awarding a research grant to undertake this project.The authors also acknowledge the Faculty of Mechanical Engineering Technology for the research facilities and space to conduct this research.

Copyright © 2022 The Authors. The substrate material and active side characteristics can affect the bubble ebullition cycle and consequently the heat transfer rate and pressure drop in microchannel evaporators. This paper presents an experimental study on flow boiling patterns, heat transfer rates and pressure drop in multi-microchannels evaporators made of copper and aluminium. HFE-7100 was used as the test fluid at atmospheric pressure, 5 K inlet sub-cooling, mass flux of 50-250 kg/m2s and wall heat flux up to 174 kW/m2. All heat sinks were made with channel width 0.46 mm, channel height 0.46 mm, giving a 0.46 mm channel hydraulic diameter. The heat sink base area was 25 mm in length and 20 mm in width. The experimental results showed that similar flow patterns were visualised for copper and aluminium namely bubbly, slug, churn and annular flow. The heat transfer coefficient in the aluminium heat sink was 12% (average value) higher than that found in the copper heat sink. The measured pressure drop in the aluminium heat sink was 28% (average value) higher compared to the copper heat sink. However, the additional pumping power required to move the fluid through the heat exchanger is small for this factor to be significant. The SEM images of the surface revealed that the number of cavities (possible nucleation sites) was higher in the aluminium surface with clear-cutting marks compared to the copper surface. This may explain the different pressure drop and heat transfer behaviour. The results of the present study indicate that aluminium heat sinks can offer comparable thermal performance to that of copper heat sinks and can also be recommended for cooling high heat flux systems. Iraqi Ministry of Higher Education and Scientific Research (MOHESR) PhD studentship; EPSRC through grant EP/K01112/1 and EP/T033045/1.

Copyright © 2022 The Author(s). This study presents the single-phase heat transfer and pressure drop characteristics of R1233zd(e) in a Brazed Plate Heat Exchanger (BPHE). Experiments on single-phase, water-to-water, were initially conducted and a correlation for the convective heat transfer coefficient of the hot water side was derived by applying the modified Wilson plot method. The experiments covered a range of Reynolds number from 80 to 1600 and Prandtl number from 2.8 to 7.0. Subsequent experiments were conducted with water-to-R1233zd(e) covering a refrigerant range of Reynolds number from 700 to 1450 and Prandtl number from 4.5 to 4.9. The results were used to assess existing correlations in the literature predicting the Nusselt number and Fanning friction factor in BPHEs. Finally, new correlations for both the hot (water) and cold (refrigerant) sides are proposed for single-phase heat transfer for this geometry covering the conditions above. The proposed refrigerant heat transfer correlation predicted 97% of all data within the ± 10% error bands at a mean absolute error value of 5.7%. EPSRC Grant EP/P004709/1.

Flow boiling heat transfer enhancement using porous coatings in microchannels has been experimentally investigated with HFE-7200. Results of the coated microchannel heat sink were compared to baseline results in a plain, micro-milled copper microchannel heat sink at similar operating conditions, namely inlet pressure of 1 bar, mass flux of 200 – 400 kg/m2 s and inlet subcooling of 10 K at wall heat fluxes between 24.5 kW/m2 to 206.6 kW/m2. Flow visualisation results and SEM surface analysis are presented. The coated surface was densely populated with well-defined cavities between 0.6 µm to 3.3 µm wide. The plain channels had fewer cavities for a given area and these were larger, i.e. up to 6 µm. Bubble generation frequency in the coated channels is significantly higher than in the plain channels due to the presence of more favourable nucleation sites on the coated surface. Flow pattern development occurred similarly in both heat sinks, namely bubbly to slug, churn and annular flow with increasing heat flux, with earlier flow pattern transitions in the coated heat sink. Enhancement in microchannel flow boiling heat transfer was shown to be influenced by mass velocity and may reach up to 44% at low heat fluxes, where the nucleate boiling mechanism is dominant. It diminishes with increasing heat flux, corresponding to nucleate boiling suppression following flow regime transition. Pressure drop increase posed by the coated heat sink is relatively small in terms of overall system power consumption but appears to be influenced by mass flux, pressure oscillations as well as channel rewetting behaviour.

Copyright © 2021 The Author(s). The pool boiling process is one of the most effective heat transfer modes capable of transferring large amounts of heat with small temperature difference between the heated surface and the fluid. In addition, fundamental knowledge of pool boiling processes is the starting point of flow boiling research and applications. It is therefore no surprise that it has been, and still is, the subject of extensive research globally for quite some time and a critical analysis is now required in order to move forward with enhanced surface designs. The current on-going research focuses on the understanding of boiling fundamentals including bubble generation, growth and bubble dynamics. In this context, fluid-surface interaction is critical. In the first part of this two-part paper we present the factors and parameters affecting the above, starting with the criteria for gas/vapour entrapment, nucleation site stability and the superheat required for heterogeneous nucleation. The models predicting the incipience superheat are critically described, classified into phase instability and superheated boundary-layer based models. This first part includes bubble growth and departure models, elucidating the effect of surface topology and wettability that can inform and facilitate the design of enhanced surfaces that are presented in Part II [10]. Three fluids of industrial interest, i.e. FC-72, HFE7100 and water were used through the discussion, as examples, to represent low and high surface tension fluids and help the understanding of surface-fluid interactions and relation to possible heat transfer enhancements. Engineering and Physical Sciences Research Council, UK (Grant Reference: EP/S019502/1).

Miniaturised electronics pose challenging thermal demands, not only at chip-level power dissipation but also at the complete system-level heat rejection, in modern electronic packages. Chip-level power densities are projected to be as high as 4.5 MW/m2 in computer systems by 2026.

Recent advancements in the electronics industry led to smaller and more powerful systems. Therefore, efficient heat dissipation for cooling of microelectronics systems, integrated circuit chips, power semiconductor devices such as IGBTs and laser diodes is required. In such systems, heat fluxes in the order of MW/m2 need to be removed from small spaces, while maintaining the temperature below a certain design limit (Karayiannis and Mahmoud in Appl Therm Eng 115:1372–1397, 2017 [1]). Flow boiling in microchannels is one of the most promising methods for achieving these high cooling demands because it can dissipate large heat fluxes over a small surface area by utilizing the latent heat of the coolant (Wang in Appl Therm Eng 110:369–381, 2017 [2]). Literature review indicates that there are still disagreements on the prevailing flow patterns, heat transfer rates, and pressure drop trends; see Mahmoud and Karayiannis (Encyclopedia of Two-Phase Heat Transfer and Flow IV, 2018, pp 233–301 [3]). The main objective of the present work was to investigate the flow boiling flow patterns, heat transfer coefficient, and pressure drop characteristics in a square metallic microchannel at different inlet subcooling, mass flux and heat flux conditions, using water as the working fluid.

The research community is currently focused on developing next generation technological solutions to utilise the energy available in domestic and industrial waste. The recoverable energy could be significant; for example, a recent study in the UK [1] identified 48 TWh/year of recoverable waste heat (17% of all industrial energy use). Organic Rankine Cycles (ORC) are best suited to harness this waste heat as they convert low and medium grade waste heat to power (

The effect of surface material on the flow boiling characteristics in a multi-microchannel evaporator is described in this paper. HFE-7100, a dielectric and eco-friendly working fluid, was tested at atmospheric pressure, inlet sub-cooling of 5 K, base heat flux up to 433.5 kW/m2 and mass flux range 50–250 kg/m2s. Two heat sinks made of copper and aluminium were fabricated having the same channel dimensions giving a hydraulic diameter of 0.46 mm with a base area of 500 mm2. The average roughness was measured and the values were 0.286 μm and 0.192 μm for the copper and aluminium, respectively. The effect of surface material was not significant at low heat flux. However, it became significant at moderate and high wall heat fluxes. The flow patterns for the two microchannels were similar and included bubble, slug, churn and annular flow. The experimental results showed that the aluminium surface achieved on the average difference of 12% higher heat transfer coefficients than those found in the copper microchannel. It also gave higher flow boiling pressure drop; the average difference was up-to approximately 30%. The different surface microstructures in the two examined heat sinks could explain the different heat transfer and pressure drop behaviour.

The 6th Micro and Nano Flows Conference was held at the Georgia Institute of Technology, Atlanta, Georgia, USA, between 9-12 of September 2018, see http://mnf2018.com. The conference Local Organizi...

Copyright © 2021 The Author(s). Heat transfer enhancement by surface modification has been extensively studied in the last twenty years. However, there remains a large discrepancy among researchers on the performance of enhanced surfaces even for the same fluid and surface preparation technique. The reasons of this discrepancy are not understood and are not discussed in past papers, including paper reviews. Part II of this two-part paper aims to present a detailed assessment of pool boiling heat transfer enhancement, relating this to Part I [1], which presented a critical assessment of fundamental concepts of heterogeneous nucleation. Current challenges in evaluating the performance of enhanced surfaces is first discussed. The performance of smooth and roughened surfaces is then discussed and the effect of fluid type is explained. Pool boiling data of two fluids, namely water and FC-72, on two enhanced substrate materials, i.e. copper and silicon were digitized and assessed in order to elucidate the reason for the discrepancy in published works and present future recommendations for heat transfer enhancement. The heat transfer enhancement mechanisms adopted by researchers were presented and critically discussed and compared. The paper contributes to the understanding of the effect of fluid-surface combinations and suggest guidelines for researchers to consider when evaluating the performance of enhanced surfaces. This will help the research community and industry to conclude on the best surface structure and surface manufacturing technique matching particular fluid of interest. Engineering and Physical Sciences Research Council, UK (Grant Reference: EP/S019502/1).

Two-phase flow heat transfer was examined in a single 5 mm inner hydraulic diameter square channel in a vertical orientation. The channel uses a resistive coating to allow for transparent heating of the walls. Transparency of the heating enabled high speed visualization of the boiling phenomena at various heat and mass fluxes. The pressure is monitored at the inlet and outlet of the channel. Infra-red thermography is used to map the external wall temperature of the channel and the local heat transfer coefficient is estimated from the local wall temperature and the saturation temperature of the liquid. Ethanol, deionized water and a 5% v/v ethanol/water mixture were used as working fluids. Three mass fluxes (0.33, 0.66 and 1.00 kg/m 2 s) were tested as well as three heat fluxes (2.8, 4.2 and 6.1 kW/m 2 ). Experiments were conducted in a controlled temperature environment, where the surrounding air was kept at 40 °C. The addition of ethanol into water (5% v/v ethanol/water mixture) was found to enhance heat transfer resulting in higher heat transfer coefficients than for either of its pure components.

Fanno theory provides an analytical model for one-dimensional confined viscous compressible flows. The model holds under the assumptions of adiabatic flow and constant cross-section channel. From theory, the differential of every flow-related quantity is expressed as a function of Mach number and friction factor. One-dimensional flow numerical models can be derived by discretizing Fanno equations. However, theory does not assess how to evaluate friction, while the model works properly only if friction is estimated correctly. Compressibility and turbulence act by deforming the velocity profile making it flatter. Assuming the friction factor function of the Reynolds number alone, in line with incompressible flow theory, is thus not correct. Better correlations should include the Mach number to address compressibility effects. Here, the impact of turbulence and compressibility on the velocity profiles in a micro-channel is analysed by means of CFD simulations. Friction factor correlations are deduced for turbulent micro-flows. The impact of the velocity profile on other quantities, such as dynamic pressure and bulk temperature, needed for the numerical model operation, is also evaluated. Additional correlations for these quantities overcome the instrinsic limits of the one-dimensional model, necessarily unaware of local velocity profiles, in a quasi-2D fashion significantly improving its predicting capabilities.

The main objective of this research was the design and test of a multi-microchannels heat sink for electronics cooling applications, which operates at system pressure near atmospheric and low mass flow rates. HFE-7100, a dielectric and eco-friendly coolant, was chosen as the working fluid. Twenty five rectangular microchannels, of width 0.7 mm and height 0.35 mm giving a hydraulic diameter of 0.46 mm, were fabricated from oxygen-free copper with a base area of 500 mm2. The channels in-between wall was 0.1 mm thick. Five mass fluxes ranging from 50 to 250 kg/m2 s were tested at fixed inlet sub-cooling near 5 K. The effect of heat flux, mass flux and vapour quality on the local heat transfer coefficient was investigated. Four flow patterns namely; bubbly, slug, churn and annular flow, were visualized using a high-speed camera mounted on a microscope. In this study, the maximum flow boiling heat transfer coefficient was 12.71 kW/m2 K at mass flux of 250 kg/m2 s. A comprehensive comparison with experimental results was conducted including flow pattern maps, heat transfer and pressure drop correlations. Some of the correlations, proposed for conventional channels and microchannels, showed good agreement with the present results.

Engineering & Physical Sciences Research Council (Grant EP/N011112/1 ).

Pressure changes caused by the growth of confined bubbles during flow boiling in mini-/microchannels lead to transient flow reversal in the presence of inlet (upstream) compressibility. A one-dimensional (1-D) model is presented to study the effect of inlet resistance on maximum flow reversal distance, local pressure fluctuations for different initial upstream compressible volumes, channel dimension, locations of nucleation site, heat flux, and initial channel velocity for water and FC-72 at atmospheric pressure and R134a at 800 kPa. The two upstream compressibility models considered are condensable vapor in a subcooled boiling region and trapped noncondensable gas.

The effect of channel aspect ratio on flow boiling characteristics (flow patterns, heat transfer and pressure drop) of HFE-7100 in copper multi-microchannel heat sinks was investigated experimentally. Three heat sinks with base area 500 mm2, channel hydraulic diameter 0.46 mm and channel aspect ratio (ratio of channel width to channel height) of 0.5, 1.0 and 2.0 were tested. The average surface roughness of the channel bottom surface was nearly the same in the three heat sinks and the measured values were 0.271, 0.286 and 0.304 µm. The local heat transfer rates were determined simultaneously with flow visualisation at mass flux ranging from 50 to 250 kg/m2s, wall heat flux from 9.6 to 191.6 kW/m2, system pressure of 1 bar and low inlet sub-cooling of 5 K. The results showed that, when the channel aspect ratio increased, the heat transfer coefficient increased, while the flow boiling pressure drop decreased. However, the heat transfer rate calculated using the heat sink base area was higher in the heat sink with the smallest channel aspect ratio, indicating an enhancement due to the largest surface area.

From Poiseuille theory, it is known that incompressible laminar fully-developed flow of a Newtonian fluid in a constant cross-section channel is characterised by steady parabolic velocity profiles after a fully-developed flow condition is attained. In turbulent fully-developed flow the velocity profiles are non-parabolic and become more flat for higher Reynolds numbers. When the incompressible hypothesis does not hold, as in the case of high velocity ideal gas flow, the velocity profile becomes flatter, as if more turbulent, due to the superposition of compressibility and turbulence effects, if applicable. This is typical in micro-channel flows, where pressure gradients are high and the gas is rapidly accelerating, eventually up to the sound velocity. As the flow accelerates the effects of compressibility grow stronger and the velocity profile keeps changing shape. The radial velocity component does not zero as in fully-developed flow but reverses after the entrance effects have damped out and grows with the Mach number. A net mass transfer toward the walls is thus generated making the velocity profile more flat. This affects the friction factor which is no longer constant, being proportional to the normal-to-wall velocity gradient, and needs to be evaluated. In the present work, the compressible friction factor is numerically investigated and correlations are proposed based on the velocity profile shape evolution as a function of the Mach number. This, together with other considerations on the velocity profile shape change, is shown to enhance the predictive capability of the Fanno theory for compressible flows.From Poiseuille theory, it is known that incompressible laminar fully-developed flow of a Newtonian fluid in a constant cross-section channel is characterised by steady parabolic velocity profiles after a fully-developed flow condition is attained. In turbulent fully-developed flow the velocity profiles are non-parabolic and become more flat for higher Reynolds numbers. When the incompressible hypothesis does not hold, as in the case of high velocity ideal gas flow, the velocity profile becomes flatter, as if more turbulent, due to the superposition of compressibility and turbulence effects, if applicable. This is typical in micro-channel flows, where pressure gradients are high and the gas is rapidly accelerating, eventually up to the sound velocity. As the flow accelerates the effects of compressibility grow stronger and the velocity profile keeps changing shape. The radial velocity component does not zero as in fully-developed flow but reverses after the entrance effects have damped out and grows with ...

Numerical simulations were performed using Fluent 14.5 to investigate single phase flow and conjugate heat transfer in copper rectangular microchannels. Two different configurations were simulated: (1) single channel with hydraulic diameter of 0.561 mm and (2) multichannel configuration consisting of inlet and outlet manifolds and 25 channels with hydraulic diameter of 0.409 mm. In the single channel configuration, four numerical models were investigated namely, 2D thin-wall, 3D thin-wall (heated from the bottom), 3D thin-wall (three side heated) and 3D full conjugate models. In the multichannel configuration, only 3D full conjugate model was used. The simulation results of the single channel configuration were validated using experimental data of water as a test fluid while the results of the multichannel configuration were validated using experimental data of R134a refrigerant. In the multichannel configuration, flow distribution among the channels was also investigated. The 3D thin-wall model simulation was conducted at thermal boundary conditions similar to those assumed in the experimental data reduction (uniform heat flux) and showed excellent agreement with the experimental data. However, the results of the 3D full conjugate model demonstrated that there is a significant conjugate effect and the heat flux is not uniformly distributed along the channel resulting in significant deviation compared to the experimental data (more than 50%). Also, the results demonstrated that there is a significant difference between the 3D thin-wall and full conjugate models. The simulation of the multichannel configuration with an inlet manifold having gradual decrease in cross sectional area achieved very reasonable uniform flow distribution among the channels which will provide uniform heat transfer rates across the base of the microchannels.

The thermoacoustic heat engine (TAHE) is a type of prime mover that converts thermal power to acoustic power. It is composed of two heat exchangers (the devices heat source and sink), some kind of porous medium where the conversion of power takes place and a tube that houses the acoustic wave produced. Its simple design and the fact that it is one of a few prime movers that do not require moving parts make such a device an attractive alternative for many practical applications. The acoustic power produced by the TAHE can be used to generate electricity, drive a heat pump or a refrigeration system. Although the geometry of the TAHE is simple, the behavior of the engine is complex with 30+ design parameters that affect the performance of the device; therefore, designing such a device remains a significant challenge. In this work, a radical design methodology using reinforcement learning (RL) is employed for the design and optimization of a TAHE for the first time. Reinforcement learning is a machine learning technique that allows optimization by specifying 'good' and 'bad' behavior using a simple reward scheme r. Although its framework is simple, it has proved to be a very powerful tool in solving a wide range of complex decisionmaking/ optimization problems. The RL technique employed by the agent in this work is known as Q-learning. Preliminary results have shown the potential of the RL technique to solve this type of complex design problem, as the RL agent was able to figure out the correct configuration of components that would create positive acoustic power output. The learning agent was able to create a design that yielded an acoustic power output of 643.31 W with a thermal efficiency of 3.29%. It is eventually hoped that with increased understanding of the design problem, in terms of the RL framework, it will be possible to ultimately create an autonomous RL agent for the design and optimization of complex TAHEs with minimal predefined conditions/restrictions.

An experimental study of flow boiling heat transfer and pressure drop was conducted using R245fa in stainless steel, brass and copper tubes of 1.1 mm internal diameter. Experimental conditions include: mass flux range 100–400 kg/m 2 s, heat flux range 10–60 kW/m 2 , pressure of 1.8 bar and exit vapour quality range 0–0.95. The tube surfaces were compared using scanning electron microscopy (SEM) and surface data acquired from confocal laser microscopy (CFLM), both showing differences between the surfaces. The heat transfer coefficient is similar in magnitude for all three materials but with a slight variation in trend. The heat transfer coefficient is seen to peak at high vapour qualities for stainless steel and brass, which is less evident with copper. The results were compared with past heat transfer correlations. These results showed better agreement with stainless steel compared to copper and brass. The pressure drop was shown to differ with surface characteristics, with the pressure drop for brass having a much steeper increase with heat flux. The pressure drop correlations tested did not show good agreement with the experimental results.

This article is available open access under a Creative Commons license (http://creativecommons.org/licenses/by-nc-nd/3.0/) Copyright © 2013 The Authors. Published by Elsevier Ltd. All rights reserved. There is a large discrepancy in the open literature about the comparative performance of the existing macro and microscale heat transfer models and correlations when applied to small/micro flow boiling systems. This paper presents a detailed comparison of the flow boiling heat transfer coefficient for R134a in stainless steel micro tubes with 21 macro and microscale correlations and models. The experimental database that was used in the comparison includes the data for 1.1 and 0.52 mm diameter tubes, mass flux range of 100–500 kg/m2 s and system pressure range 6–10 bar obtained in the course of this study. The effect of the evaporator heated length on the comparative performance of the correlations and models was investigated using three different lengths of the 1.1 mm diameter tube (L = 150, 300 and 450 mm). This comparative study demonstrated that none of the assessed models and correlations could predict the experimental data with a reasonable accuracy. Also, the predictability of most correlations becomes worse as the heated length increases. This may contribute in explaining the discrepancy in the comparative performance of the correlations from one study to another. A new correlation is proposed in the present study based on the superposition model of Chen. The database used in developing the correlation consists of 5152 data points including the current experimental data and data obtained previously with the same test rig, fluid and methodology for tubes of diameter 4.26, 2.88, 2.01 mm. The new correlation predicted 92% of the data within the ±30% error bands with a MAE value of 14.3%.

The experimental results of two-phase flow boiling pressure drop of R134a in vertical microdiameter stainless steel tubes are presented in this paper. The tests were conducted using four tubes: one tube with an inner diameter of 0.52 mm and 100 mm heated length and three tubes with an inner diameter of 1.1 mm and different heated lengths (150, 300, and 450 mm). Other experimental conditions include mass flux range of 200–500 kg/m2-s, system pressure range of 6–10 bar, inlet subcooling value of about 5 K, and heat flux range of 1–140 kW/m2. The results indicated that the total measured two-phase pressure drop increases with increasing mass flux, heat flux (exit quality) and decreasing system pressure and tube inner diameter. The test section heated length was found to have a significant effect on the measured pressure drop per metre length. The total measured two-phase pressure drop results were also compared with eighteen macro- and microscale models and correlations.

A non-thermal plasma reforming unit operating at atmospheric pressure has been developed for converting methane to CO x -free hydrogen. Argon was used to provide additional electrons and photons for higher reaction rates. A series of experiments was performed for positive corona discharge at a fixed inter-electrode distance (15 mm) to study the effects of discharge power (range of 14–20 W) and residence time (60, 120, 180 and 240 s). A second series of experiments studied the effect of inter-electrode distance on hydrogen production, with distances of 15, 20, 25, 30 and 35 mm tested. The analysis of the results shows that both discharge power and residence time, have a positive influence on methane conversion, hydrogen selectivity and energy conversion efficiency. Longer discharge gaps favour hydrogen production. A final series of experiments on corona polarity showed that a positive discharge was preferable.

In Celebration of Professor John W. Rose BScEng PhD DScEng(Lond) CEng FIMechE FASME on his 80th birthday

In this study, we survey the alternative approach of using plasma technology to produce CO x free hydrogen. In a corona discharge reactor operating at atmospheric pressure, electrons were energized by an electric field to ionize propane and induce chemical reactions. A range of each test parameter was covered, namely the effect of power input in the range of 4.5–105 W and discharge time of 1, 2, 3, 4, 5 and 8.18 min. A 19% hydrogen content by volume was achieved at a power input of 102 W and discharge time of 8.18 min. Copyright , Oxford University Press.

Accurate predictions of two-phase pressure drop in small to micro-diameter passages are necessary for the design of compact and ultra-compact heat exchangers, which find wide application in process and refrigeration industries and in the cooling of electronics. A semimechanistic model of boiling two-phase pressure drop in the confined bubble regime is formulated, following the three-zone approach for heat transfer. The total pressure drop is calculated by time-averaging the pressure drops for single-phase liquid, elongated bubble with a thin liquid film, and single-phase vapor. The model results were compared with experimental data collected for a wide range of tube diameters (4.26, 2.88, 2.02, 1.1, and 0.52 mm) for R134a at pressures of 6–12 bar. In this model's present form, its predictions are close to those of the homogeneous flow model but it provides a platform for further development.

Boiling heat transfer in small diameter tubes has been experimentally investigated using R134a as the working fluid. The heat transfer experiments were conducted with two stainless steel tubes of internal diameter 4.26 and 2.01 mm respectively. Other parameters were varied in the range: mass flux 100–500 kg/m2 s; pressure 8–12 bar; quality up to 0.9; heat flux 13–150 kW/m2. It was found that the nucleate boiling is dominant when the vapour quality is less than about 40–50% for the 4.26 mm tube and 20–30% for the 2.01 mm tube. Above these quality values, heat transfer coefficients decrease with vapour quality. Furthermore, this decrease occurs for the entire quality range at high heat flux values. Flow visualization experiments were carried out using the same experimental facility with Pyrex glass tubes. A flow pattern map was obtained at a system pressure of 10 bar and tube diameter of 4.26 mm.

An experimental study of low-level turbulence natural convection in an air filled vertical square cavity was conducted. The cavity was 0.75 m high × 0.75 m wide × 1.5 m deep giving 2D flow. The hot and cold walls of the cavity were isothermal at 50 and 10 °C respectively giving a Rayleigh number of 1.58 × 109. The local velocity and temperature were simultaneously measured at different locations in the cavity and both mean and fluctuation quantities are presented, i.e. ū, u′rms, v, v′rms, T , T′rms, u ′ v ′ , u ′ T ′ and v ′ T ′ . The local and average Nusselt numbers, the wall shear stress as well as the turbulent kinetic energy and the dissipation rate of the temperature variance are also presented. The experiments were conducted with very high accuracy and as such the results can form experimental benchmark data and will be useful for validation of computational fluid dynamics codes.

The energy performance in buildings is a complex function of the building form and structure, heating system, occupancy pattern, operating schedules, and external climatic conditions. Computer simulations can help understand the dynamic interactions of these parameters. However, to carry out a multi-parameter analysis for the optimisation of the building energy performance, it is necessary to reduce the large number of tests resulting from all possible parameter combinations. In this paper, the lattice method for global optimisation (LMGO) for reducing the number of tests was used. A multi-parameter study was performed to investigate the heating energy use in office buildings using the thermal simulation code APACHE (IES-FACET). From the results of the sensitivity analysis it was possible to estimate the relative importance of various energy saving features.

An experimental study of low level turbulence natural convection in an air filled vertical square cavity was conducted. The dimensions of cavity were 0.75 m × 0.75 m × 1.5 m giving two-dimensional flow. The hot and cold walls of the cavity were isothermal at 50 and 10°C, respectively, giving a Rayleigh number of 1.58 × 109. The temperature and velocity distribution was systematically measured at different locations in the cavity, and was nearly anti-symmetrical. An experimentally obtained contour plot of the thermal field and a vector plot of the air flow in the cavity are reported for low turbulence natural convection in such cavities for the first time. The wall shear stress and the local and average Nusselt numbers are also presented. The Nusselt number compares well with previous results; the agreement on the velocity and temperature profiles at mid-height near the vertical walls is fair. Differences were found at mid-width and in the rate of velocity and temperature changes near the walls. The experiments were conducted with high accuracy. Therefore, the results can form experimental benchmark data and will be useful for CFD code validation.

A new degree-day model for estimating energy consumption in buildings has been developed that removes the need for correction factors and allows the user more control over the inputs. The proposed method incorporates a model for determining mean internal temperatures of buildings. The model has been tested for accuracy and sensitivity to input variations. It has also been possible to quantify the uncertainty in the model, which allows the user to determine the usefulness of a result to a particular application.

A formal method has been developed for quantifying the uncertainties in degree-day-based energy estimates in buildings. Different interpretations of base temperature as a means of describing the building energy balance were investigated, with the result of an improved degree-day model that does not require the use of correction factors. This model has been tested using high-quality data from building simulations, and the outputs have been subjected to statistical uncertainty analysis. The results show that degree-day uncertainties diminish with longer time frames. The uncertainties also impact on regression techniques used in monitoring and targeting of building performance, and these uncertainties may exceed typical targets for energy efficiency improvements.

Simple expressions have been developed for important variables of the flow in an open-flow solar regenerator (with applications in absorption cooling systems) through a first- and second-law analysis of the system. In particular, expressions for the regenerator figure of merit, the exergetic or rational efficiency and the entropy production rate have been given and applied to some available data with reasonably good results.

A three-dimensional (3-D) code which can predict the complete performance of high efficiency boilers is described in this paper. The model is in two parts. The combustion chamber is modelled in the first part and the heat exchanger in the second. The thermal efficiency is calculated from the predicted results. The computational model was based on the two-dimensional (2-D) TEAM code which has been extended into 3-D and extensively modified by the present authors. In the code, the gaseous combustion was modelled by Simple Chemical Reaction System concept; the Radiation process was handled by the ‘Discrete Transfer’ method. The modelling results of the combustion chamber were used as input for the calculation of the consequent finned tube heat exchanger. The heat exchanger calculation included conduction, convection and radiation. Predictions were performed for a low Reynolds number boiler and compared with existing data. Good agreement was obtained.

Integral fin tubes are commonly used in the condensers of refrigeration, air conditioning and process industries especially where low surface tension fluids are used. In the last few decades, several three dimensional (3D) enhanced surfaces were developed for condensation heat exchangers. Also, several improvements were introduced to the standard integral finned tubes which resulted in a performance comparable to that of the 3D enhanced surfaces. Some of these improvements include optimisation of the fin specifications and the fin profile to make use of the surface tension and gravity forces, the use of ribs on the surface of the fins and the inclusion of drainage techniques. A review of the experimental and theoretical work used to augment the performance of integral finned tubes is presented in this paper. This is followed with a review of the 3D surfaces developed and a comparison between the integral fin tubes and the 3D surfaces. Performance in bundles and the effect of vapour shear are specific issues that were considered in the comparison.

The results of an experimental study of electrohydrodynamic (EHD) enhancement of pool boiling are presented in this paper. A high intensity dc electric field was applied to R123 and R11 boiling on the shell side of a smooth five-tube shell and tube heat exchanger. The tubes were electrically heated and were arranged in three rows. The electrode system carrying the high voltage comprised fourteen mild steel rods placed in between and around the tubes. Experiments were performed at heat flux values of 20–5 kW m −2 and applied voltages of 0–25 kV. The enhancement ratio for R123 increased linearly with an applied voltage of up to 15 kV and was strongly dependent on heat flux. Increasing the voltage beyond 20 had no effect on the enhancement ratio at heat flux values greater than 10 kW m −2 . At lower heat flux, some further increase in the enhancement ratio was obtained when increasing the voltage from 20 to 25 kV. The enhancement with R11 was marginal, indicating the importance of fluid properties to electrohydrodynamic enhancement. The effect of pressure on the enhancement was also examined.

The uncertainties associated with calculating degrees-days from traditional formulae and other approximate methods were examined. Comparisons were based on calculations using hourly temperature records. A definition of uncertainty for monthly degree-days and some typical errors in annual values are presented. The ranges of uncertainty are small for published degree-days, but could be significant for a small percentage of occasions. If the methods are used for base temperatures lower than the current standard then the uncertainties increase. Also shown are the effects of these errors on the regression analysis often used in building energy monitoring.

Experimental results for etectrohydrodynamic (EHD) effects on nucleate boiling at passively enhanced surfaces, such as Thermoexcel-HE and Gewa-T, are reported. The experiments were carried out using a single-tube test rig by applying a 0- to 30- kV dc potential to a dielectric liquid (Refrigerant-114). The EHD effect on the enhanced surfaces is compared with results of earlier experiments on smooth and Thermoexcel-C tubes and discussed. The results indicate that the boiling hysteresis at these passively enhanced surfaces can be eliminated and the nucleate boiling can be further enhanced and controlled by applying an electric field. An explanation of bubble initiation at low superheat by an electric field is offered in terms of the effect of the field on bubble critical radius.

The electrohydrodynamic (EHD) enhancement mechanism is first outlined in this paper. A comprehensive review of past work on EHD single and two-phase heat transfer, as well as past work in the related area of EHD-induced flow, is then presented. Correlation attempts are also reviewed. Recent experimental results for EHD boiling and condensation in single- and multi-tube heat exchangers are discussed. A description of the possible practical EHD electrode systems for applications in power production cycles and regrigeration is also presented. The research work needed to clarify outstanding questions in EHD and encourage its use in practical systems is discussed.