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A low-speed wind tunnel was designed to experimentally study the cooling performance of a novel arrangement of wet medium for evaporative cooling of air. In the wet medium arrangement (CELdek7060), a 300 mm cooling section was formed by 100 mm wet medium + 100 mm gap + 100 mm wet medium, which will be denoted as 300 mm medium-gap-medium. Normal arrangement was experimentally studied for comparison with medium thicknesses of 100 and 300 mm, respectively. The result shows that the pressure drop of the 300 mm medium-gap-medium is lower than that of the normal medium at thicknesses of 300 mm and is close than that of 200 mm within the tested air speeds, and the cooling effectiveness of the 300 mm medium-gap-medium is higher than that of the normal medium at 200 mm but lower than that of the normal medium at 300 mm. Meanwhile, the correlations of pressure drop and heat transfer coefficient for the 300 mm medium-gap-medium are developed based on the test data. This study proves that the proposed 300 mm medium-gap-medium is feasible and shows superior performance when compared with normal medium at 200 mm thickness.

Latent heat thermal energy storage is essential for a broad range of multidisciplinary thermal applications, due to its capability of keeping a relative constant temperature during thermal energy storage/release. The aspect ratio (AR) and installation angle (tilted angle) for a latent heat energy storage tank play important roles in addressing the issue of thermal energy storage/release efficiency. In this work, the coupled effects of tank aspect ratio and tilted angle upon melting phase change were both experimentally and numerically investigated. Wide range of tank aspect ratios from 0.1 to 8.0 was studied with respect to four representative tilted angles (0°, 30°, 60°, and 90°). The detailed features of melting front propagation, temperature distribution, and free convection development in the melt phase were quantified. Results indicated that melting heat transfer was markedly affected by the tilted angle, regardless of tank aspect ratio. However, the fastest melting occurred at different tilted angles, if different tank aspect ratios were considered. With AR 2.0, the fastest melting was achieved at an inclination of 60°.

Turbulent heat and fluid flow of a nanofluid in an annulus with sudden area reduction are numerically and experimentally studied. Two different nanofluids are considered. Nanoparticles made of aluminum oxide (Al2O3) and titanium oxide (TiO2) with volume concentrations varying from 0.005 to 0.02 have been examined. Reynolds number and heat flux varied from 10,000 to 40,000 and 1000 to 6000 W m−2, respectively. Area reduction ratios from 1 to 2 were studied in this research. For turbulence modeling, the SST k–ω model in a 3D domain was utilized. In order to validate the numerical results, Al2O3–water-based nanofluid was tested, and a great agreement was observed. It was noted that the maximum thermal performance enhancement was about 194.7% in an annular channel with an area reduction ratio of 2 compared with a straight pipe using water. The heat transfer enhancement is attributed to the downstream flow separation and recirculation region. It was noted that rising the volume concentration of the nanoparticles improves the heat transfer coefficient; this enhancement was about 26.9% (Al2O3) and 5.5% (TiO2). Also, the influence of the Reynolds number on the surface heat transfer coefficient augmentation is quantitatively reported in this paper. The maximum pressure drop was about 7.5% for Al2O3 and 5.9% for TiO2 nanofluid, compared with pure water at a reduction ratio of 2 at the highest Re value considered here.

Solar enhanced natural draft dry cooling tower (SENDDCT) is a novel heat rejection device for thermal power plant, which uses solar energy to reheat the air after the heat exchanger and therefore improves the ventilation of the tower for performance enhancement. This paper develops 3-D models to compare the effects of circular/sawtooth arrangements of heat exchanger on cooling performance of a 140 m high SENDDCT. The result shows that sawtooth arrangement can improve the ventilation and heat dissipation rate of SENDDCTs. Under no crosswind condition, the flow field distributions of sawtooth and circular arrangements are similar in the tower. The ventilation and heat dissipation rate of sawtooth arrangement increases by 34.0 % and 46.8 % respectively when compared with circular arrangement under no crosswind condition. Under crosswind condition, the flow field of sawtooth arrangement is less affected by crosswind than that of circular arrangement. The ventilation rate and heat dissipation rate of sawtooth arrangement increases by 38.5 % and 65.0 % respectively when compared with circular arrangement at crosswind speed of 3 m/s. This study verifies that the sawtooth arrangement can weaken the crosswind effect and improve the cooling performance of SENDDCT and provides guidance for the optimal arrangement of heat exchanger for the SENDDCT.

Swirling turbulent plumes are investigated with the Morton, Taylor, and Turner model (MTT) and the $\mathrm{\ensuremath{\Gamma}}$-approach. We derive the asymptotic solutions using regular perturbation methods, and discuss the influence of swirling motions on the plume properties for lazy and forced plumes, respectively. Specially and intriguingly, by introducing a swirl, a forced plume can even be turned into a lazy one in the near field during the vertical evolution.

Extensive computational investigations have been performed to obtain more detailed information about the peculiar phenomena of turbulent supercritical carbon dioxide (sCO2) flow as ideal heat trans...

Transient free convection from a heated surface immersed in a cold ambient is investigated using scale analysis. The heated plate is our model for the heat exchangers in a natural draft dry cooling tower. The scale analysis results led to a timescale for initiation of the plumes. Furthermore, a model is developed to study the near-wall free convection which allows the prediction of plume extension and merging process. Both the normal and lateral components of the fluid velocity are expressed as functions of the Prandtl and Rayleigh number. Expressions are proposed to predict the plume size and spacing in the lateral direction. Results are then successfully compared with independent experimental data available in the literature.

This paper experimentally studies the air-side flow resistance of three different water collecting devices (WCDs) being U-type, V-type and rectangle-type. A wind tunnel was designed to simulate the airflow across the WCDs within air speeds of 0.5–3.0 m/s. The effect of dry/wet conditions on the air-side flow resistance was also investigated. The results show that the air-side flow resistance of the rectangle-type WCD is the highest among all while those of U-type and V-type are more or less identical. Therefore, the U-type WCD is recommended for industrial application as it can collect more water with low air-side flow resistance. Meanwhile, wet condition produces higher air-side flow resistance when compared with dry condition for all of the studied WCDs. Finally, a correlation is developed to express air-side pressure drop as a function of air speed, air density, water-to-air volumetric flow rate ratio and non-dimensional shape parameter which has been introduced for predicting the air-side flow resistance across WCDs.

This paper investigates the effects of tube arrangement and pitch on the performance of a single row bundle of bare tubes. A transient simulation is conducted to analyze flow and heat transfer characteristics of this problem. Velocity fields obtained from numerical simulation are validated against those obtained using particle image velocimetry in this work. Heat transfer and pressure drop for the flat bundle are found to be in good agreement with available data in the literature. In addition to flat bundles, two cases of inclined bundles, i.e., A and V frames, are investigated. Compared to a flat bundle, while these two cases lead to lower heat transfer rate by about 30% (maximum), the pressure drop of a flat bundle can be higher by about 60%. Finally, a correlation for predicting the drag coefficient as a function of maximum (bundle) velocity is presented.

Purpose In this study, closed-form solutions are presented to investigate thermohydraulics of liquid films in a rotating heat pipe. The film thickness is expressed as a function of flow rate. Design/methodology/approach Further, sensitivity of both film thickness and flow rate to the length of the rotating heat pipe can now be investigated using the explicit expressions presented here. Findings To make it easier for practical application, an approximate solution is presented on top of the exact solution. Originality/value Both approximate and exact solutions are then applied to note that results are in good agreement when compared to those available in the literature.

In exploring the suitability of natural draft dry cooling towers (NDDCTs) for dispatchable thermal power plants, the transient start-up process of NDDCTs is investigated numerically. It is an established fact that the draft caused by the density difference between the interior and outside air drives the tower flow at steady-state conditions. The study finds that, starting from cold, the tower flow goes through three stages before the steady-state is reached. In the first stage, natural convection is dominant. The stagnation of thermal plumes before pinch-off contributes to a sharp increase in the inlet air temperature and the merger of rising caps further increases the rate at which the interior is heated. As the inside air warms up, an inside-outside density difference emerges and the draft starts contributing to the flow development in addition to the plumes. This is the second stage characterized by mixed convection. The distribution of air temperature inside tower becomes horizontally uniform and vertically linear through the mixed convection stage. In the third stage, the draft finally becomes the dominant mechanism and is sufficient to explain subsequent transition to steady-state and onwards. The dimensionless number, Richardson number (Ri) is introduced to identify the boundaries between these three stages. The cold air incursion is observed as a parallel cyclical phenomenon with impact on the tower flow development. A general criterion for steady state is proposed that accounts for cold air incursion. The results are validated against the experimental data from the University of Queensland natural draft dry cooling tower Gatton test rig and compared with the results of a simplified theoretical model. A better understanding of the start-up process through the numerical investigation presented in this paper provides insights to how to reduce the NDDCT transient response and thereby improving the dispatchability of the CST system employing the NDDCT.

This paper computationally investigates the turbulent heat transfer of sCO flows cooled in large horizontal tubes with diameter of 15.75 mm, 20 mm and 24.36 mm using RANS turbulence models. The numerical models were validated against experimental data published in literature to demonstrate the reliability of CFD simulations on the heat transfer coefficient prediction and buoyancy effect capture to turbulent sCO. Based on the validated model, a number of computations, involving a wide range of operating conditions, have been carried out. The effect of mass flux (200–800 kg/ms), pressure (8–10 MPa), heat flux (5–36 kW/m) and tube diameter has been analysed. Results demonstrate that the AKN model shows the best consistencies with the experimental measurements and is also able to well reproduce the heat transfer characteristics under various conditions. As the mass flux increases, the heat transfer coefficients go up due to the enhanced turbulence diffusion. Pressure has a significant effect on the distribution of heat transfer coefficient, and its peak drops sharply with rising pressure. At T > T, with the heat flux and tube diameter increasing, sCO heat transfer performance is improved; whereas at T < T, the heat flux and tube diameter almost have no effects on the heat transfer performance. Considerable deviations with the existing heat transfer correlations necessitate the development of a new correlation to predict the heat transfer coefficients of cooling turbulent sCO in large horizontal pipes. Based on the reliable computational datasets, a Nusselt number equation based on the Gnielinski form with the ratio of density incorporated is formulated.

Direct air-cooling is favourable for the supercritical CO2 power cycle; however, the heat transfer behaviour of supercritical fluids when cooled near the critical point in large tubes is uncertain. Physical property changes of CO2 near the critical point may lead to buoyancy effects, and the direction of external air flow over a finned-tube heat exchanger may impact the rate of heat transfer. A novel low-speed wind tunnel which can rotate between horizontal and vertical orientation has been designed and constructed to investigate supercritical CO2 aircooled heat exchangers. The design differs from typical wind tunnels to achieve the required test section size, manoeuvrability and space constraints. Air velocity and turbulence characteristics have been measured using hot wire anemometry and show the wind tunnel generally achieves the design criteria.

In combustors, high-pressure fuel injector nozzles are employed for liquid atomization and spray generation. The internal flow within nozzles, especially cavitation, plays an important role in promoting the breakup of liquid jets. The formation mechanism and evolvement of cavitation are reviewed and described. Different cavitation regimes were characterized experimentally and the positive effect of cavitation on liquid atomization was confirmed. Different empirical formulas correlating the discharge coefficient with fluid-dynamical parameters are summarized. The applicability, advantages, and disadvantages of two popular computational models for flow modeling, i.e., the interface tracking model and the homogeneous equilibrium model, are reviewed. The effects of cavitation on the generated spray are discussed.

This paper presents the results of computational investigations on cooling heat transfer of turbulent sCO2 in three horizontal tubes with diameter of 15.75 mm , 20.00 mm and 24.36 mm using RANS turbulence models at a pressure of P = 8.0 MPa . Four models with good prediction performance demonstrated in literature (RNG k - e model and three other low-Reynolds number k - e models of LS, YS and AKN) have been validated against experimental measurements and to observe that results from the AKN model are closer to experimental data. Details of heat transfer behaviour of sCO2 cooled in horizontal tubes within this diameter range are revealed and the influence of heat flux, tube diameter and buoyancy on heat transfer performance have been discussed. Results demonstrate that at T b > T pc (pseudocritical temperature), sCO2 heat transfer performance is enhanced as the heat flux and tube diameter increase; whereas at T b T pc , the heat flux and tube diameter almost do not affect the heat transfer performance. The buoyancy effect only generates slight enhancement for turbulent heat transfer from sCO2 flowing in horizontal tubes with large diameters. However, as the values of Richardson number Ri that quantifies the buoyancy effects continue increasing within Ri > 0.1 , the buoyant force is enhanced, which in turn impairs the heat transfer near T pc . This is a result contrary to past reports confined to small diameter tubes, which is mainly attributed to the accumulation of denser cold fluids at the bottom of the pipe when buoyancy effects are strong.

Numerical and experimental studies have been performed to investigate the macroscopic spray structure and spray characteristics of sprays generated by a full-cone pressure swirl atomizer. The simulation employs Eulerian-Lagrangian scheme to account for the multiphase flow and the linearized instability sheet atomization model to predict film formation, sheet breakup and atomization. Reynolds-Averaged Navier–Stokes (RANS) equations are solved for turbulent gas flow. The model predictions show great consistency with the experimental measurements of the spatial variation of the droplet size and velocity obtained from Phase Doppler Particle Analyser (PDPA). The robustness of this model makes it useful to predict the structures and characteristics of co-flow sprays produced by pressure-swirl atomizers. This particular spray is quite important in spray cooling application but is not extensively studied. The study reveals that the entrainment effect and intense central-region atomization cause small droplets to concentrate on the spray axis and large droplets to dominate in the peripheral region of the spray. This finding is consistent with the observation that turbulence kinetic energy of air is maximum near the nozzle exit, where atomization is intense and momentum exchange is strong, and gradually decreases in both radial and axial directions. Moreover, the drops inside the full cone are relatively small, and evaporate more easily than their large counterparts in the peripheral region, hence removing substantial sensible heat from surrounding air and creating low-temperature region in the central of the spray.

In thermal power cycles including concentrating solar thermal (CST) plants, natural draft cooling towers (NDCTs) are widely used heat-dumping facilities. One inherent drawback of NDCTs is that their cooling performance can be compromised by changes in ambient conditions, particularly temperature, which inevitably reduces the net power output of the cycles. Current methods resolving this issue are limited in a few options including inlet air pre-cooling, exit air heating, and fan assistance, each with considerable operational or initial cost. To more economically reduce energy efficiency losses of the power cycles due to inefficient cooling, this paper proposes a new concept of swirling plume method for both dry- and wet-type NDCTs. The method is to rotate the plume strongly like a tornado in the tower upper part and above the towers to increase the overall tower updraft capacity (pressure). The swirling plume is induced by high-speed air jets distributed at certain locations using a much smaller flow rate. A numerical investigation on a 20 m-tall dry-type NDCT model has been conducted verifying that this concept increases the airflow and the water temperature drop of the heat exchanger by at least 53.6% and 3.57 °C (39.2%), respectively, under 35 °C ambient temperature. This cooling performance enhancement enables a half megawatt-scale sCO2-based CST power cycle to recover its net power output, by 4.98%, to the level almost same as that at 30 °C ambient temperature. The air jet to create such a swirling plume consumes only 1/7 of the recovered power roughly. Compared with a traditional fan-forced cooler working under exactly the same condition, this concept requires significantly smaller energy in long-term operations as it would run only during temperature extremes. A simplified analytical modelling has found that the cooling tower performance is improved due to that the swirling plume creates an equivalent extra draft height on top of the tower which is attributed to two different vortical effects. The overall angular momentum of the swirl is a critical factor in these effects.

Building upon fractal theory and relying exclusively on analytical models, we develop models for predicting the permeability and effective thermal conductivity of two-directional (2-D) fibrous porous materials. In contrast to previous permeability and conductivity models, two hypothetical parameters ( β and ζ ) with physical meaning are introduced to consider the contact thermal resistance at the fiber interfaces, and analytical models for both in-plane and out-of-plane directions are developed. Relevant geometrical and physical parameters, including porosity, average tortuosity, and thermal resistance, are obtained by modeling the representative structure (RS) of the fibrous porous material. Good agreement with existing experimental data for fibrous materials over a wide range of porosity (from 0.50 to 0.99) validates the developed models, for both the permeability and effective thermal conductivity. It is demonstrated that, compared with previous models based on simplified geometries relying on periodic distribution assumptions, the current fractal model can better characterize the randomness of pore size and distribution commonly found in commercial fibrous materials.

The main objective of the present work is to enhance the stability and accuracy of Volume-of-Fluid (VOF) method for simulating multiphase flow problems with large density ratios and moving interfaces. In order to accomplish this, a novel high-order TVD flux-limiter scheme is developed and applied to approximate the convective fluxes in the momentum, energy and volume fraction equations. Moreover, the classical implicit non-iterative PISO algorithm is modified according to SIMPLER algorithm and the combined model (PISOR) is then employed to handle the pressure-velocity coupling. Furthermore, the piecewise linear interface calculation (PLIC) based on the Efficient Least Squares Volume-of-Fluid Interface Reconstruction Algorithm (ELVIRA) is adopted for the treatment of the interface and curvature estimation. The robustness and consistency of the proposed modifications in handling violent free-surface flows involving interface rupture and coalescence are verified via simulation of several canonical test cases including: dam break over the dry and wet beds, rotating square patch of fluid and Rayleigh-Taylor Instability (RTI). The results show that, the proposed flux-limiter scheme can significantly suppress false-diffusion errors and maintain interface sharpness while preserving the boundedness and positivity of the volume fraction field. Moreover, it is found that the proposed PISOR model has robust convergence behavior in strongly coupled multiphysics problems and provides more stable and consistent results than the standard PISO and SIMPLER algorithms. The performance and efficiency of proposed models are further verified, for single phase flow, through analysis of transient entropy generation due to natural convection heat transfer in differentially heated cavity.

Natural draft dry cooling tower (NDDCT), with little water usage, is a primary choice for power plants in dried regions. However, the increased ambient temperature during summer days decreases the cooling performance of NDDCT. Inlet air pre-cooling is used to alleviate the tower deterioration by making use of water evaporation to remove excess heat from inlet air. To achieve the maximal cooling effect, the injection heights, radial distances and injection directions of employed nozzle LNN1.5 were studied based on the CFD results. The study shows that lower nozzle placement can cool the central part of the radiator while the higher one cools the middle part. Additionally, the increasing extended length can boost the evaporation process of generated spray. Moreover, the upward and co-flow injections have poorer performance than the downward and counter-flow injections. Furthermore, an introduction of wall cover changes the flow field and drives the pre-cooled air flow through the edge of radiator. Since the wall cover reduces the blockage caused by the near-wall vortex the resultant low-temperature region move outwardly.

Buoyancy has been found to have a significant influence on the flow and heat transfer behaviors of turbulent sCO2. This paper uses the computational method to investigate the flow and heat transfer characteristics of turbulent sCO2 in a large horizontal tube with the buoyancy effects taken into account, RNG and three selected low-Reynolds number k - e turbulence models have been validated against experiments published in literatures. Using the validated CFD model, the buoyancy mechanisms affecting sCO2 flow and heat transfer within large horizontal tubes are revealed, and the effect of the heat flux have been analyzed. Comparison against experimental results suggests that AKN low-Reynolds number model exhibits the best prediction. Buoyancy influences the flow structure and turbulence levels mainly via the induced secondary circulation. Buoyancy effects are stronger at increased heat flux values. The secondary circulation becomes pronounced at higher heat flux levels and increases the temperature difference between the top and bottom tube surfaces. In large horizontal tubes, slight heat transfer enhancements in the mixed convection are observed near the pseudocritical point. However, a significant deterioration is found at higher heat load density. This is a result contrary to past reports confined to small diameter tubes.

This paper offers numerical and experimental analysis of forced convection through an annulus filled with aluminium foam. Effects of flow rate and foam pore density on the performance of the heat exchanger were investigated. Specifically, 5 and 20 pore per inch (PPI) aluminium metal foams were tested at three different airflow rates; 20, 85 and 150 standard litre per minute. In parallel, the problem has been simulated numerically. Once validated against experimental data, numerical simulations were conducted to add to the level of details obtained from experiments. The thermal study was done by analysing the temperature field throughout the porous volume and determining the thermal entrance length. This parameter, the thermal entrance length, establishes a reliable design criteria for metal foam-filled heat exchangers, since it marks the length beyond which heat transfer does not significantly increase while the pressure drop keeps growing.

It is demonstrated that analytical models of permeability and effective thermal conductivity for open-cell metallic foams can be developed using fractal theory, for the theory lends itself better to the problem at hand where the pore morphology of the foam is typically random. Upon determining the average tortuosity based on selected representative flow streamlines in a representative structure (RS) of the foam, the permeability as a function of porosity, average tortuosity and pore size ratio is analytically obtained, with no empirical or fitting parameter needed. Similarly, the analytical fractal model of effective thermal conductivity does not include any empirical or fitting parameter. Good agreement with experimental data verifies both models, squarely justifying the applicability of using fractal theory to characterize open-cell metallic foams. It is also demonstrated that the present fractal models can better characterize the randomness of pore distributions than previous permeability and conductivity models built upon simplified geometries including periodically distributed unit cells.

This paper shows the promising capabilities that the aluminum metal foams offer in enhancing the water pool boiling on flat surfaces. Different metal foam samples, with pore density ranging between 5 and 40 Pore Per Inch (PPI), with similar porosity values, around 0.92, and two different thickness values, 5 and 10 mm, were tested to investigate their pertinent pool boiling performance. The saturated boiling curves at atmospheric pressure were obtained. Furthermore, using a high speed camera, heat and fluid flow inside and above the porous layer were investigated. The results showed that boiling heat transfer can be greatly enhanced by the use of metal foams partly because of an earlier onset of the nucleate boiling. Moreover, over a wide range of operating conditions foams lead to remarkably higher heat transfer coefficients, compared with a smooth aluminum reference surface. For a more comprehensive understanding of the problem, we tried to correlate the existing data in the literature for different foam geometries and base materials. However, the existing correlations and independent test data in the literature do not agree well; not even within the same order of magnitude. Hence, in order to offer a generic solution, predictions based on the results of an artificial neural network (ANN) are sought. A large database, comprising 758 experimental data points available in the open literature, was collected and then used to develop, train and validate a model based on ANN to estimate the heat transfer performance of metal foams during water pool boiling. Our data show that the predicted Nusselt number is within 10% of the measured experimental data. Moreover, we demonstrate that the developed ANN tool can be successfully implemented to predict the intrinsic complexity of water pool boiling inside metal foams that in most cases cannot be articulated by merely relying on conventional semi-empirical methods.

Pre-cooling of inlet air using water sprays is a promising approach to enhance the performance of natural draft dry cooling towers. The present article investigates the usage of saline water in spray assisted natural draft dry cooling towers. A CFD model is developed to predict droplet evaporation, transport, and wet length (distance from the nozzle where all droplets are dry due to evaporation). This length is a key parameter when utilizing saline water in dry cooling tower designs as the risk of corrosion due to the existence of Cl − ions and deposition of salt should be minimized. Experimental tests are conducted in a wind tunnel at cooling tower representative conditions to validate the model. A good agreement is observed between numerical and experimental results. Once validated, results from a numerical experiment incorporating both non-uniform droplet distributions typical for sprays and spray half angle were used to develop a general dimensionless correlation for wet length in horizontal ducts, based on effective diameter and flow conditions. The dimensionless correlation presented in this work allows the influence of operating and ambient parameters on the wet length to be predicted. Air temperature has the strongest influence on wet length. A 4 °C increase in temperature leads to 11.4% reduction in wet length. Other operating parameter, such us droplet diameter and air velocity, have weaker effects.

Thermal dispersion effects on fully developed forced convection inside a porous-saturated pipe are investigated. The pipe wall is assumed to be kept at a uniform and constant heat flux. Having the fully developed velocity field furnished by an arbitrary power series function, the energy equation is solved using asymptotic techniques for the limiting case when thermal conductivity, as a result of thermal dispersion, weakly changes with the Peclet number. A numerical solution, valid for the entire range of thermal dispersion conductivity, is also presented. This latter solution is presented to check the accuracy of the former. The two solutions are then cross-validated in the limits. Besides, results are found to be in good agreement with those previously reported in the literature.

Porous materials (media) are solids that are permeated by a network of pores. The interconnection of the pores allows the flow of one or more fluids through the material. Porous materials exist widely in nature and made industrially. Sandstone, limestone, rye bread, wood, and the human lung could be exemplified as natural porous media. Man-made porous media include metallic and non-metallic foams, porous ceramics and packed bed, which are widely used in industrial applications. Metallic foams are mostly used in heat exchangers, and packed beds are utilized for thermal storage applications. The Introduction of Chapter 1 deals with the fundamental concepts of fluid flow and heat transfer in porous materials. It covers a wide range of aspects of porous media from porosity and permeability to some unresolved issues in modeling of transport in porous media, including interface modeling between clear (fluid) region and solid region, heat flux splitting, and heat flux bifurcation phenomenon. For fluid flow in the media, some comparison between experimental and theoretical studies is conducted revealing that for a high Reynolds number (more than unity), the well-known Ergun relation could not fit well with experimental data. Convective and radiative heat transfer in porous materials are presented. Both local thermal equilibrium (LTE) and local thermal non-equilibrium (LTNE) models for convection in the media are discussed.

This chapter deals with free convection of incompressible Newtonian fluids in porous media. It is presumed that the reader is familiar with the basics of convection in porous media. A non-exhaustive survey of the literature on free convection in porous media is presented mainly to show the progress in the field and also to highlight the gaps. Suggestions as to how some of these gaps might be filled are put forward. The chapter skips over some details and instead focuses on the application of some techniques that can be used to simplify analyzing complex practical problems. Some examples are presented to demonstrate the implementation of the proposed techniques to obtain reasonably accurate results. Furthermore, samples of the obtained results, compared to those obtained from more sophisticated and involved solution procedures, are presented.

Swirling motions have been proven to improve the thermal performance of short natural draft dry cooling towers by reducing the cold air inflow and increasing the draft speed. However, crosswind influences on the favourable swirl effects have not been investigated. To fill this gap, 3-Dimensional simulations of a short natural draft dry cooling tower are carried out. Three different locations of the swirl generator, with solid body rotation, are compared under windless and crosswind conditions. The results show that, with no wind present, introducing swirling motions right above the heat exchangers is found to be the optimal location for improving around 40 % of the reduced thermal performance, which is casued by cold air inflow penetration. On the other hand, as the swirl intensity further increases after the cold air inflow is eliminated, locating it at the tower outlet performs the best on the air draft speed enhancement, and thus further increases approximately 17 % of the heat transfer rate at the angular frequency input of 2 s - 1 . In the presence of crosswind and windbreak walls, air flows through the heat exchangers and tower non-uniformly. By mounting the swirl generator right above the heat exchangers, the uniform index of the heat flux can be improved by 5 % with 1 s - 1 angular frequency input. More importantly, inducing swirls at the tower outlet is still the optimal choice for increasing the air draft speed through the tower. With 2 s - 1 angular frequency input, the thermal performance of the tower can be enhanced by 11 ~ 17 % in accordance with the crosswind speed.

To accelerate the start-up period of Gatton NDDCT, windbreak walls are introduced under the heat exchanger inspired by theoretical analysis, which suggests quicker start-up with increasing values of β, which is a factor that represents the proportion of crosswind redirected into the tower via the heat exchanger by windbreak walls. Three-dimensional (3-D) numerical models at three different wind angles of attack and at crosswind ranging from 1 m/s to 15 m/s produce results consistent with the theoretical analysis. The start-up time with crosswind always follows the similar trend, i.e., first increasing to the peak and decreasing monotonously until a critical speed, beyond which the start-up time keeps almost constant. The air flows through individual heat exchanger bundle due to the interaction between the natural draft (due to heating) and the crosswind effects. These effects are resolved separately at each bundle location at the acting wind speed. Finally, optimum orientations to accelerate the start-up duration are numerically computed. When the wind is mild at up to 4 m/s, windbreak walls are better designed at a 60° wind angle of attack. For crosswind speeds from 5 to 8 m/s, 0° windbreak walls have a better performance. Finally, for wind speeds over 9 m/s, the 30° windbreak walls seem to be most effective. These results should be applicable to any relatively short NDDCT and also imply that at sites with non-uniform wind conditions, a facility to change the windbreak walls orientation with respect to the wind speed would be useful.

A new draft equation involving swirling motions for natural draft dry cooling towers (NDDCTs) is derived and solved analytically. A 2-D axisymmetric model for a short NDDCT is built and computational fluid dynamics (CFD) simulations are carried out to verify the theoretical predictions. Results show that the optimized location for swirl input is at the tower outlet to avoid the swirl decay. It is noted that the swirl influence on the air flow draft velocity gradually becomes significant especially when the dimensionless input swirl ratio exceeds 2. The theoretical predictions generally agree with the numerical results, but deviate when: (1) vortices adjacent to the wall occur in the presence of excessively strong swirl, in the case with swirl created by a thin source zone; (2) cold air inflow penetrates due to the significant heat exchanger resistance coefficient; (3) vortex breakdown appears, in the case with swirl filling the whole tower. All the aforementioned unfavourable phenomena are local effects and thus cannot be predicted by the draft equation unless proper local resistance terms are added.

Free convection in a partially foam-filled enclosure is theoretically investigated. Two independent approaches are undertaken. Scale analysis has been conducted along with a thermal resistance network model to predict the performance of metal foams as heat sinks to remove heat from a horizontal heated plate. Results from scale analysis were used to predict the flow regime transition from that of the viscous drag-dominated to that of form-drag dominated flow. Predictions from the two approaches are then cross-validated and compared against available experimental data in the literature. A good degree of agreement was observed. Finally, using the theoretical models, a parametric study was conducted to investigate the effects of different parameters on the Nusselt number.

This work presents a theoretical approach, based on momentum and energy or Reynolds analogy, to investigate forced convection in microducts of arbitrary cross-section. H1 boundary condition is assumed for gas flow in the slip-flow regime with further complication of a temperature jump condition assumption. It is shown that applying an analogy concept, one can relate the slip-flow results to those of no-slip/no-jump ones available in the literature. Present results for slip flow in microchannels of parallel plate, circular, triangular, trapezoidal, polygonal, rhombic, and rectangular cross-sections are found to be in close agreement with those in the literature. A further modification, based on Chilton-Colburn analogy is applied to enable the Prandtl number variation effects when the Prandtl number is not equal to unity.

A natural draft dry cooling tower rejects heat in a power plant. Spray cooling of the inlet air to the cooling tower improves the total efficiency of the power plant. To overcome the scarcity of natural water sources, this research is studying the usage of saline water in spray assisted dry cooling towers. A nozzle is analysed experimentally. It is shown that the CFD model captures the spray well. A full cone spray is simulated in a vertical cylindrical domain representative of cooling tower flows. To investigate the influences of initial and ambient conditions on the spray performance, fourteen different cases are simulated and trends analysed. It is shown that the distances from the nozzle, after which the dry stream starts (wet lengths), are in the range of 4.3–5.25 m depending on the test conditions. A dimensionless study is performed on the wet length and cooling efficiency as the two main parameters. Finally, to predict the wet length and cooling efficiency, two dimensionless correlations are presented and their impact on cooling tower operation is discussed.

Thermal effects on the dynamics and stability of the flow past a circular cylinder operating in the forced convection regime is studied experimentally for Reynolds numbers (Red) between 1000 and 4000, and different cylinder wall temperatures (Tw) between 25 and 75 °C by means of Particle Image Velocimetry (PIV). In each experiment, to acquire 3000 PIV image pairs, the temperature and Reynolds number of the approach flow were held constant. By adjusting different temperatures in different Reynolds numbers, the corresponding Richardson number was varied between 0.0 and 0.2. With increasing temperature of the wall cylinder, significant modifications of the wake flow pattern and wake vortex shedding process were clearly revealed. By increasing the Richardson number, the high temperature gradient in the wake shear layer creates a type of vorticity with opposite sign to that of the shear layer vorticity. This temperature gradient-vorticity weakens the strength of the shear layer vorticity, causing delay in reaching the recreation point. In addition to the wake characteristics, it is found that, as the Richardson number is increased, the organization of the vortex shedding is altered and the relative position of the first detached vortices with respect to the second one is changed. This change varies the frequency of the shedding process.

Purpose – The purpose of this paper is to improve the volume-averaged models for free convection flow in porous media. Design/methodology/approach – A pore scale simulation is conducted against which an independent volume-averaged solver is fine-tuned. Findings – Micro and macro scale results can merge if proper choice of local thermal non-equilibrium and thermal dispersion models are selected. This depends on the range of Ra values investigated. Originality/value – This is the first time a work like this is published in the literature.

Thermal dispersion effects on fully developed forced convection inside a porous-saturated parallel plates duct with uniform heat flux at the wall are investigated on the basis of the Brinkman model. Having known the velocity distribution, the energy equation is solved using asymptotic techniques for the limiting case when thermal conductivity, as a result of thermal dispersion, weakly changes with the Peclet number. A numerical solution, valid for the entire range of thermal dispersion conductivity, is also presented. This latter solution is performed to check the accuracy of the former one. It was seen that in the limiting regions the results of both methods are in close agreement with each other and also with those previously reported in the literature. Validity range for the presented closed form solution is also discussed as a function of the Peclet number.

This article investigates the performance of saline water, compared to pure water in spray cooling and demonstrates the existence of several advantages. To simulate the crystallisation behaviour of saline water droplets, a set of modifications are made to the multicomponent discrete phase model (DPM) of ANSYS FLUENT. After validation against single droplet data, a practical spraying application with a single nozzle in a vertical flow path is studied. The results are compared with a similar case using pure water as the coolant. It is shown that using saline water for spray cooling improves cooling efficiency by 8% close to the nozzle. Furthermore, full evaporation takes place substantially earlier compared to the pure water case. The mechanism behind this phenomenon is explained. The consequence of this is a reduction of up to 30% in the distance between nozzle and the creation of a dry gas stream. This paper provides new fundamental understanding in the area of saline spray cooling, and shows that the use of saline water can lead to a number of benefits, such as reduced water costs (compared to pure fresh water), reduced infrastructure costs (more compact cooling towers), and improved cooling performance.

Turbulent convection heat transfer of upward fluid flows in vertical annular channels with uniformly heated inner wall and adiabatic outer wall is investigated numerically. Two concentric geometries with outer-to-inner diameter ratios of 2.4 and 3 as well as two eccentric geometries with identical outer-to-inner diameter ratio of 2.4 and eccentricities of 0.25 and 0.5 are investigated. Heat transfer deterioration, similar to that of a circular pipe, is observed. The range of buoyancy parameter, over which deterioration is observed, is close to that of circular pipes. Moreover, it is observed that when the value of eccentricity increases, the deterioration phenomenon becomes less pronounced and recovery of heat transfer coefficient occurs earlier, i.e. at lower buoyancy parameters compared to concentric annuli.

Crosswind effect is a common issue which limits the cooling efficiency of natural draft dry cooling towers (NDDCTs) of all sizes. On short NDDCTs with total heights less than 30 m, this effect might be much more significant. Following the authors’ previous numerical investigation on crosswind effects in a 15 m-tall cylindrical NDDCT, an experimental study was carried out and is presented in this paper. The study used a 1:12.5 scaled cooling tower model equipped with an electric resistance heater simulating horizontally placed heat exchangers. The air velocity, temperature, and the heat input on the model were measured at different crosswind speeds in a wind tunnel. Comparisons against CFD models show good agreement between the experimental and numerical results when the similarity conditions between the CFD model and the experimental model are fully satisfied. Based on these results, the total heat transfer rate of NDDCTs was proposed to be a combination of a natural convective heat transfer term and a forced convective one. In small cooling towers, the natural convection term is comparable with the forced convection term. This explains why the correlation of the total heat transfer with the wind speed has a turnabout point below which the heat transfer decreases with increasing crosswind speed and above which it does the reverse. The turnabout point occurs when the sum of natural and forced convection terms is the minimum.

A parametric study has been conducted to investigate particle deposition on solid surfaces during free convection flow in an internally heated and cooled square cavity filled with air. The cavity walls are insulated while several pairs of heaters and coolers (HACs) inside the cavity lead to free convection flow. The HACs are assumed to be isothermal heat source and sinks with temperatures Th and Tc (Th > Tc). The problem is numerically investigated using the Eulerian–Lagrangian method. Two-dimensional Navier–Stokes and energy equations are solved using finite volume discretization method. Applying the Lagrangian approach, 5000 particles, distributed randomly in the enclosure, were tracked for 150 s. Effects of drag, lift, gravity, buoyancy, pressure gradient, shear stress terms, thermophoresis and Brownian forces on particles movements are considered. Furthermore, effects of various design parameters on the heat transfer rate and deposition of particles such as Rayleigh number ( 10 4 ⩽ Ra ⩽ 10 7 ) as well as orientation and number of the HACs are investigated. Our simulations indicate that thermophoretic force can significantly affect the distribution of particles of d p = 1 μm diameter. It is also found that at low Rayleigh numbers the particle distribution is strongly non-uniform. Moreover, it was observed that by increasing number of the HACs and changing orientation of the HACs from vertical to horizontal, deposition rate of the solid particles increases significantly.

Numerical simulation of a geothermal reservoir, modelled as a bottom-heated square box, filled with water-CO2mixture is presented in this work. Furthermore, results for two limiting cases of a reservoir filled with either pure water or CO2are presented. Effects of different parameters including CO2concentration as well as reservoir pressure and temperature on the overall performance of the system are investigated. It has been noted that, with a fixed reservoir pressure and temperature, any increase in CO2concentration leads to better performance, that is, stronger convection and higher heat transfer rates. With a fixed CO2concentration, however, the reservoir pressure and temperature can significantly affect the overall heat transfer and flow rate from the reservoir. Details of such variations are documented and discussed in the present paper.

In this paper the flow structures downstream of a foamed tube are compared to those of a bare tube with the same frontal areaand length. Experiments were conducted in a wind tunnel at Reynolds numbers 4000 and 16000. Particle image velocimetry (PIV) was used to obtain velocity vector field in two different perpendicular planes. To measure the effect of flow three-dimensionality, field of divergence on both planes was obtained and compared with each other. Moreover, to characterize the size of the flow structures downstream of the tube, for each of the aforementioned cases, two-point correlation functions were used as the statistical analysis tool. Analysis showed that, compared to the bare tube, the foamed one, in the X-Z plane (stream-wise, span-wise) increases the three-dimensionality of the flow which is set forward in the perpendicular plane despite being less pronounced. Moreover, the structures downstream of a foamed tube are elongated in the stream-wise direction and are linearly independent of the inlet velocity, however, in span-wise and normal direction no significant change in the size of the structures between bare and foamed tube has been observed.

This paper presents a numerical model for understanding particle transport and deposition in metal foam heat exchangers. Two-dimensional steady and unsteady numerical simulations of a standard single row metal foam-wrapped tube bundle are performed for different particle size distributions, i.e. uniform and normal distributions. Effects of different particle sizes and fluid inlet velocities on the overall particle transport inside and outside the foam layer are also investigated. It was noted that the simplification made in the previously-published numerical works in the literature, e.g. uniform particle deposition in the foam, is not necessarily accurate at least for the cases considered here. The results highlight the preferential particle deposition areas both along the tube walls and inside the foam using a developed particle deposition likelihood matrix. This likelihood matrix is developed based on three criteria being particle local velocity, time spent in the foam, and volume fraction. It was noted that the particles tend to deposit near both front and rear stagnation points. The former is explained by the higher momentum and direct exposure of the particles to the foam while the latter only accommodate small particles which can be entrained in the recirculation region formed behind the foam-wrapped tubes.

Heat and mass transfer to and from a single solution droplet is studied in this work. A new theoretical model to predict the evaporation behaviour of solid containing water droplets is presented. The model, implemented in MATLAB, is used to predict the process of droplet evaporation with prediction results successfully validated against data from the literature. Also, an experimental study was performed to study the evaporation of a single droplet containing NaCl and water. To investigate the influence of concentration, tests were performed with droplet having initial radiuses of approximately 0.5 mm and initial mass concentrations of 3% and 5%. Results obtained from the developed model were found to be in good agreement with our experimental data. Finally, it was shown that the current model, allowing for a smooth transition from surface evaporation to crystallisation, is able to simulate the process more accurately compared to existing models in the literature which lead to a, less realistic, sharp transition.

A simple theoretical model, validated against available numerical and experimental data in the literature, is presented to predict the effects of crosswind on the performance of natural draft dry cooling towers. The intersection of asymptote method, along with scale analysis, is used to find a closed-form solution for the airflow rate at the tower exit for given crosswind speeds. The total heat rejected under a windy condition is then calculated based on the air mass flow rate at the tower exit. This theoretical model allows for parametric studies and can generate accurate data. Interestingly, the model results, expected to be accurate within an order of magnitude, are more accurate than anticipated when compared to available experimental and numerical data in the literature. In fact, the maximum relative error is observed to be 15% when current theoretical predictions are compared to available experimental data. The results of this study will be useful for future work on the development of air-cooled con...

Based on slip flow at pore level a theoretical model is presented to predict the gas permeability and thereby the overall pressure drop for flow through a porous medium. The model maps the porous structure to a number of parallel micro-channels of arbitrary but constant cross-sectional shapes which remains uniform along the flow path. The gas permeability is found to follow Klinkenberg's equation. The Klinkenberg's slip factor is obtained as a function of matrix porosity and no-slip permeability as well as gas properties. Results are generalized by assuming an arbitrary polygonal shape for the pores. The proposed methodology is simple to follow and easy to implement. Theoretical predictions are then compared to existing experimental data in the literature to observe good agreement.

This study describes a detailed numerical model of large diameter finned tube heat exchangers for a dry-cooled supercritical carbon dioxide (sCO2) Brayton cycle. Multiple-row heat exchangers are discretised into two-dimensional cells, and physical properties and heat transfer coefficients are evaluated locally at each cell to account for the large changes in sCO2 properties near the critical point. Air-flow is modelled as either forced draft, or natural draft within a natural draft dry cooling tower (NDDCT) model. Conditions anticipated for sCO2 Recompression cycle heat rejection are used to determine the heat exchanger area required for the cycle. The influence of different correlations for sCO2-side heat transfer is studied, showing that up to 27% more heat exchange area is predicted using generic-fluid heat transfer correlations when compared to those developed specifically for sCO2. The model is used to determine the effect of design ambient temperature on the heat exchanger size, as well as heat exchanger performance at elevated off-design ambient temperatures. Criteria for the presence of buoyancy effects in sCO2 flow from literature are shown to be exceeded by an order of magnitude under the conditions modelled, demonstrating that uncertainty remains regarding of the behaviour of sCO2 heat transfer under these conditions.