Condensation of Refrigerant R404a in channels of diameter 0.5–3 mm is investigated. The specific focus of this work is to obtain quantitative information on flow mechanisms using image analysis techniques on high speed video. Experiments are conducted on condensing R404a at mass fluxes ranging from 200 to 800 kg m −2 s −1 at vapor qualities of 0.05 to 0.95. The experiments are conducted at the high pressures representative of actual operation of air-conditioning and refrigeration equipment, instead of extrapolating findings for adiabatic air-water mixtures at atmospheric pressure to high pressure refrigerant condensation. Pressure drops are also measured as a function of geometry and operating conditions during the flow visualization experiments. Quantitative image analysis enables detailed computation of void fraction and vapor bubble parameters such as frequency, velocity, diameter and length with a high degree of repeatability and with low uncertainties. The effect of operating conditions on the flow patterns, void fractions, and vapor bubble parameters for saturation temperatures ranging from 30 to 60°C is documented. The resulting void fraction models provide closure for pressure drop and heat transfer models, which has thus far not been possible in the reported works in the literature.

In this paper experimental results of pressure drop and heat transfer for different ceramic sponges (variables: material, ppi number and porosity) are presented. The pressure drop data is correlated using an Ergun-type equation, which yields to an accuracy of 20%. The heat transfer coefficients were determined by a transient method and correlated in dimensionless form (Nusselt number) by a power law of Reynolds and Prandtl number. Furthermore the applicability of the well known Generalized Le´veˆque Equation (GLE) on solid ceramic sponges is checked. The GLE provides a correlation between pressure drop and heat transfer. The investigation shows that the tendency observed for the data corresponds with that of the theoretical model.

This paper presents simulations for a jet impinging against a flat plane covered with a layer of a porous material. Macroscopic equations for mass, momentum and energy, for the fluid and for the porous matrix, are obtained based on the volume-average concept. The numerical technique employed for discretizing the governing equations was the control volume method with a boundary-fitted non-orthogonal coordinate system. The SIMPLE algorithm was used to handle the pressure-velocity coupling. The effect of porosity and energy model on the local distribution of Nu was analyzed. Results indicate that for low porosity materials, a substantially different Nu number is calculated depending on the energy model applied.

The effective heat and mass transport properties of a porous packed bed of particles undergoing a high-temperature solid-gas thermochemical transformation are determined. The exact 3D geometry of the reacting porous media is obtained by high-resolution computer tomography. Finite volume techniques are applied to solve the governing conservation equations at the pore-level scale and to determine the effective transport properties as a function of the reaction extent, namely: the convective heat transfer coefficient, permeability, Dupuit-Forchheimer coefficient, tortuosity and residence time distributions. These exhibit strong dependence on the bed morphological properties (e.g. porosity, specific surface area, particle size) and, consequently, vary with time as the reaction progresses.

In order to simulate heat and mass transfer in porous media whose scales range micron to nanometers, this work intends to provide a scheme for flow field simulation for such porous media. Since Navier-Stokes equations are no longer applicable to high Knudsen number (Kn) flow regimes, the conventional lattice Boltzmann method (LBM) cannot be applicable to flows in nanoscale porous media. Hence, a modified lattice Boltzmann method is applied for computing flows in micro-nano porous media in the transitional flow regimes at moderately high Knudsen numbers. The lattice Boltzmann equation applied is an extended version using an effective relaxation time associated with Kn and a regularization procedure coupled with Maxwell’s diffuse-scattering boundary condition for walls. For the flow field where the representative molecular mean free path varies (effectively the Kn varies locally), the locally defined Kn is introduced. In order to verify the LBM scheme, the results are compared with those of the molecular dynamics (MD) simulations by the Leonard-Jones potential. The flow fields considered are in modeled nano-porous media whose porosity is around 0.9. The results of micro-nanoscale porous media flows at Knudsen numbers: Kn = 0.04–0.24 show reasonable agreement in both the simulation methods and confirm the reliability of the presently applied LBM. Interestingly, in complex flow geometry, the advantage of higher order discrete velocity models of the LBM is not notable. Therefore, it is concluded that conventional discrete velocity models, say the D2Q9 and D3Q19 models are reasonably enough for flows in micro-nanoscale porous media.

Thermal transport phenomena in porous media are characterized by conduction through solid matrix and filling gas, and also by radiation. The gas is dispersed in the porous system depending on the pore size distribution. In each pore, the gas contributes to the heat transfer between the pore surfaces. This effect is strongly influenced by pore size, gas atmosphere, accommodation coefficient and other factors. A recent publication of the present authors focused on modeling the change of the effective thermal conductivity when the gas atmosphere is changed. In the current contribution, the effect of pore size distribution on heat transfer in macro, micro, and nano insulation materials is presented. Samples were chosen from five different highly porous materials with different pore size distribution within the macro, micro, and nano classes. Porosity and pore size distribution of the samples were chosen to get a clear characterization of the materials. The effective thermal conductivity was measured by applying the radial heat flow method at temperatures up to 1000 °C. Evaluating Knudsen effect from the pore size distribution alone does not give plausible explanation for the measured thermal conductivity. However, it is important to consider the kind of connections between the pores. In case of nano materials, the radiation effect proves to be strongly dependent on the Knudsen number.

This paper reports the cross-plane thermal conductivity of amorphous and crystalline templated mesoporous titania thin films synthesized by evaporation-induced self-assembly. Both sol-gel and nanocrystal-based films were considered, with respective average porosities of 30% and 35%. The pore diameter ranged from 7 to 25 nm and film thickness from 60 to 370 nm while the average wall thickness varied from 3 to 25 nm. Nanocrystals in crystalline mesoporous films featured diameters between 9 and 13 nm. The thermal conductivity was measured at room temperature using the 3ω method. The experimental setup and the associated analysis were validated by comparing the thermal conductivity measurements with data reported in the literature for dense titania films with thickness ranging from 95 to 1000 nm. The cross-plane thermal conductivity of the amorphous mesoporous titania thin films did not show strong dependence on pore size, wall thickness, or film thickness. This can be attributed to the high atomic scale disorder of amorphous materials. Heat is thus mainly carried by localized non-propagating vibrational modes. The average thermal conductivity of the amorphous mesoporous titania films was identical to that of the nanocrystal-based films and equal to 0.37 W/m.K. Thermal conductivity of sol-gel crystalline mesoporous titania thin films was significantly larger than that of their amorphous counterparts. It also depended on the organic template used to make the films. The results indicated that the pore size was not an important factor. Instead thermal conductivity depended only on porosity, crystallinity, nanocrystal size and connectivity.

This paper reports the room temperature cross-plane thermal conductivity of pure silica zeolite (PSZ) MEL and MFI thin films. PSZ MEL thin films were prepared by spin coating a suspension of MEL nanoparticles in 1-butanol solution onto silicon substrates followed by calcination and vapor-phase silylation with trimethylchlorosilane. The mass fraction of nanoparticles within the suspension varied from 16 to 55%. This was achieved by varying the crystallization time of the suspension. The thin films consisted of crystalline MEL nanoparticles embedded in a non-uniform and highly porous silica matrix. They featured porosity, relative crystallinity and MEL nanoparticles size ranging from 40 to 59%, 23 to 47% and 55 to 80 nm, respectively. PSZ MFI thin films were made by in-situ crystallization, were b-oriented, fully crystalline and had a 33% porosity. Thermal conductivity of the PSZ thin films was measured at room temperature using the 3 ω method. The cross-plane thermal conductivity of the MEL thin films remained constant around 1.02 ± 0.10 Wm −1 K −1 despite increases in (i) relative crystallinity, (ii) nanoparticle size and (iii) yield as the nanoparticle crystallization time increased. Indeed, the effect of increases in these parameters on the thermal conductivity was compensated by the simultaneous increase in porosity. PSZ MFI thin films were found to have the same thermal conductivity as MEL thin films even though they had smaller porosity. Finally, the average thermal conductivity of the PSZ films was three to five times larger than that reported for amorphous sol-gel mesoporous silica thin films with similar porosity and dielectric constant.

Introducing the interfacial pressure jump terms based on the surface tension into the momentum equations of two-phase two-fluid model, the system of governing equations is turned mathematically into the hyperbolic system. The eigenvalues of the equation system become always real representing the void wave and the pressure wave propagation speeds as shown in the present authors’ reference: Numerical Heat Transfer —Part B, vol. 40, pp. 83–97 . To solve the interfacial pressure jump terms with void fraction gradients implicitly, the conventional semi-implicit method should be modified as an intermediate iteration method for void fraction at fractional time step. Owing to this modified numerical scheme with surface tension effect, the advanced semi-implicit method (ASIM) then becomes stable without conventional additive terms. As a consequence, including the interfacial pressure jump terms with the advanced semi-implicit method, the numerical solutions of typical two-phase problems can be more stable and sound than those calculated exclusively by using any other terms like virtual mass, or artificial viscosity.

Biomorphic porous SiC composite ceramics were produced by chemical vapor infiltration and reaction (CVI-R) technique using paper preforms as template. The thermal conductivity of four samples with different composition and microstructure was investigated: a) C-template b) C-SiC, c) C-SiC-Si 3 N 4 and d) SiC coated with a thin layer of TiO 2 . The SiC-Si 3 N 4 composite ceramic showed enhanced oxidation resistance compared to single phase SiC. However; a key property for the application of these materials at high temperatures is their thermal conductivity. The later was determined experimentally at defined temperatures in the range 298–373K with a laser flash apparatus. It was found that the thermal conductivity of the porous ceramic composites increases in the following order: C-template < C-SiC < C-SiC-Si 3 N 4 < SiC-TiO 2 . The results were interpreted in regard to the porosity and the microstructure of the ceramics.

A capacitive void fraction sensor was developed to study the dynamic behavior of refrigerant two-phase flows in horizontal macro-scale smooth tubes of evaporators used in air-conditioning and heat pump applications. A first prototype was successfully tested with air-water flow. The dielectric constant of liquid HFC refrigerants however is much smaller than the one of water, which results in much smaller capacitances to be measured. Therefore an improved version of the capacitance transducer was developed to make dynamic measurements of these HFC fluids possible. The sensor signal can be used for flow pattern detection and void fraction measurements of high pressure HFCs. A macro-scale test facility for two-phase flow and heat transfer studies of high pressure HFCs was designed and constructed. The adiabatic test section consists of a sight glass with high speed camera and the capacitive void fraction sensor. Two datasets of sensor signals were gathered using R410A and R134a respectively (G = 200–500kg/m 2 s and x from 0 to 1 in steps of 0.025 at T sat = 15°C). To find more objectivity in flow pattern mapping, a signal analysis was performed. The average, the variance and a high frequency parameter were found suitable for flow regime classification into slug flow, intermittent flow and annular flow by using the fuzzy c-means clustering algorithm. A probabilistic flow pattern map is presented for the HFC data. These maps clearly quantify the width of the transition zones and can be applied for probabilistic heat transfer and/or pressure drop modeling.

Studies on two-phase flow in small scale pipes have become more important, because of the application of mini-scale devices in several engineering fields including, high heat-flux compact heat exchangers, and cooling systems of various types of equipment. In a mini pipe the behavior of two phase flow is not the same as flow in conventional pipes. The difference is caused by different effective forces; for e. g. inside a mini pipe capillary forces are more important in comparison with gravitational forces. This paper is devoted to numerical simulation of gas-liquid two phase flow in a vertical mini pipe. Prediction of bubble shape and the effects of gas and liquid velocities on flow characteristics are considered. Also simulation involves prediction of changes in average void fraction along pipe axis. Numerical simulations in this paper are performed by a designed and developed CFD package which is based on Eulerian-Eulerian approach. The governing equations which are solved in the CFD package are momentum, continuity and Fractional Volume of Fluid (VOF) function equations. The fluid is assumed to be viscous and incompressible. The pressure-velocity coupling is obtained using the SIMPLEC algorithm. The geometry, which have been studied in this paper, is a D = 1.02 mm pipe, with 500 mm height. Bubble shape and the distribution of void fraction in a mini pipe are related to many parameters such as: gas and liquid velocities, pressure losses and etc. Since these mechanisms vary over time, time-average value of void fraction is used. Comparisons between Numerical results and experimental work which performed by hibiki et al. [1] indicated good agreement. Also results have shown that the present model is capable to simulate the behavior of nitrogen-water two phase flow in a mini pipe with acceptable accuracy. Furthermore, the results indicates that average void fraction along the pipe axis is related to the height and nitrogen superficial velocity. Also it is observed that at constant nitrogen superficial velocities, average void fraction decreases with water superficial velocity increments.

Multilayer porous gas enthalpy-radiation converter consisting of two porous plates divided by free space is proposed to enhance the performance of single layer porous converter. The energy equations of the gas and the solid phases are employed by using separate energy equations model, and the convective heat transfer between the two phases based on an empirical volumetric heat transfer coefficient proposed by Kamiuto and San San Yee is estimated. The P 1 approximation method is employed to evaluate the radiation of transfer equation. Results show that high porosity and PPI (pores per inch) are effective to raise the gas enthalpy-radiation conversion. The upstream porous layer should be lower in porosity and PPI than the downstream porous layer. To increase the temperature drop across the porous gas enthalpy-radiation converter, the inlet gas temperature should be high and the upstream radiation temperature should be low. To obtain maximum temperature drop across the multilayer porous converter, both layers should be made of pure absorbing porous materials. In case the upstream radiation temperature exceeds an inlet gas temperature, however, use of pure scattering porous plate on the upstream side and pure absorbing porous plate on the downstream side will obtain the maximum temperature drop across the converter. The validity of the predicted numerical model also has been confirmed by the experimental data.

Heat and mass transfer in a permeable fabric system used for Personal Protective Equipment (PPE) was investigated using hot air jet impingement conditions to mimic the jet exhaust of Short Take-Off and Vertical Landing (STOVL) aircraft. The STOVL aircraft uses a thrust-vectoring nozzle of the jet engine and a lift fan in order to vertically land and take off a short runaway. The jet engine exhaust is a new kind of thermal hazard for military personnel operating within an affected zone of the jet exhaust. An experimental approach was used to measure the thermal response of a fabric system consisting of permeable fabric samples and air pocket using a high-speed jet impingement. The jet impingement conditions consisted of two different temperatures: one of 100°C and another of 200°C at a jet impingement velocity of 32 m/s. Air was used as the working fluid. In this study, two permeable fabrics, (NOMEX IIIA and Cotton) commonly used for the Personal Protective Equipment (PPE) were investigated. The physical properties (porosity, permeability, Ergun coefficient, and density) and the thermo-physical properties (thermal diffusivity, thermal conductivity, and specific heat) of the fabrics were measured. A one-dimensional, two-medium formulation assuming thermal non-equilibrium between solid (fabric) and gas (air) phases in the fabric layer was used for the numerical analysis. The measurement results from the fabric experiment were used to define boundary conditions and adjust various heat transfer correlations and input data used in the numerical model. The experimental and numerical results of the temperatures of the fabric system were compared. The effects of the air temperature of the jet impingement on the thermal response of the fabric system were discussed.

In this study, the effective thermal conductivity (ETC) of sintered loop heat pipe wicks, with pure nickel powders, pure copper powders, Ni-10wt%Cu powders and Ni-20wt%Cu powders were experimentally investigated. The ETC of sintered Ni-Cu wicks is found less than those of sintered pure nickel wick and sintered pure copper wicks. In the same porosity level, addition of copper into nickel will reduce ETC of the sintered Ni-Cu wicks. The sintered Ni-20wt%Cu wick presents the lowest ETC among the tested wick samples. Compared to experimental results, Alexander model can provide a reasonable prediction in some wick samples.

Thermal barrier coatings (TBCs) are applied to blades, vanes, combustion chamber walls, and exhaust nozzles in gas turbines not only to limit the heat transfer through the coatings but also to protect the metallic parts from the harsh oxidizing and corrosive thermal environment. There is a growing interest in operating these hot gas path (HGP) components at optimal conditions which has resulted in a continuous increase of the turbine inlet temperatures (TITs). This has resulted in the increase of heat load on the turbine components especially in the high pressure side of the turbine necessitating the need to protect the HGP components from the heat of the exhaust gases using novel TBC such as electron beam physical vapor deposition thermal barrier coatings (EBPVD TBCs) and Air Plasma Sprayed thermal barrier coatings (APS TBCs). This study focuses on the estimation of temperature distribution in the turbine metal substrate (IN738) and coating materials (EBPVD TBC and APS TBC) subjected to isothermal conditions (1573 K) around the turbine blade. The heat conduction in the turbine blade and TBC systems necessary for the evaluation of substrate thermal loads are assessed. The steady state 2D heat diffusion in the turbine blade is modeled using ANSYS FLUENT computational fluid dynamics (CFD) commercial package. Heat transfer by radiation is fully accounted for by solving the radiative transport equation (RTE) using the discrete ordinate method. The results show that APS TBCs are better heat flux suppressors than EBPVD TBCs due to differences in the morphology of the porosity present within the TBC layer. Increased temperature drops across the TBC leads to temperature reductions at the TGO/bond coat interface which slows the rate of the thermally induced failure mechanisms such as CTE mismatch strain in the TGO layer, growth rate of TGO, and impurity diffusion within the bond coat.

A numerical model based upon a Monte Carlo Ray Tracing (MCRT) code was developed to predict the thermal radiative properties (reflectance, transmittance, emittance) of a family of virtual alumina ceramics with prescribed textural features. The numerical samples were constituted of polydisperse spherical pores [20–120 μm] embedded in a continuous and homogenous alumina matrix. The texture of the alumina ceramics were numerically settled, allowing, in our case, to fix their porosity, their volumetric surface and their radii size distribution. The MCRT code was applied at T = 300 K and T = 1300 K since, at the local scale, the complex indices of refraction of the alumina matrix were known. The comparisons of the emittance spectra of the numerical samples with the ones acquired by infrared emittance spectroscopy on a single crystal of alumina (T = 300 K and T = 1300 K) allow to discuss the effects played by the temperature and by each textural parameters.

A general and simple model for simulating the steady behavior of air-to-refrigerant fin-and-tube evaporators, which accounts for detailed flow state inside the tubes, is introduced. To account for the heat transfer between air and the working fluid, the evaporator is divided into a number of control volumes. Space dependent partial differential equations group is obtained from the mass, energy and momentum balances for each one. The corresponding discretized governing equations are solved afterwards. Empirical correlations are also required to estimate the void fraction, the internal and external heat transfer coefficients, as well as the pressure drops. According to the phase of refrigerating fluid, the evaporator can be divided into two distinct zones on the refrigerant-side: the vapor zone and the two-phase zone, while special emphasis is performed on the treatment of the two-phase zone. The distribution of flow pattern has been evaluated with the aim of improving the calculation accuracy. The model prediction is validated against experimental data for an evaporator using R22 as the working fluid, which shows a reasonable level of agreement: the cooling capacity is predicted within the error band of 3%. The developed model will have wide applications in operational optimization, performance assessment and pipeline design.

Two-phase flow on the shell side of a shell and tube heat exchanger is complex. Several studies have produced flow pattern maps that show surprising differences in flow regime boundaries for data sets that contain relatively small variations in fluid and flow properties. Despite this, correlations for void fraction and pressure drop are sufficiently accurate to allow the thermal-fluid design of heat exchangers to be completed. However, these correlations are based on experimental data taken from tube bundles containing tubes with diameters less than 20 mm. This study examines their applicability to tube bundles containing tubes with a diameter of 38 mm. Results for void fraction and pressure drop are presented for air-water flows near atmospheric pressure. The results were obtained for flows through a thin-slice, in-line tube bundle containing 10 rows. The tube bundle contained a central column of tubes with half tubes placed on the shell wall to simulate the presence of other columns. The tubes were 38 mm in diameter and 50 mm long with a pitch to diameter ratio of 1.32. Previous studies have shown that the void fraction in a shell-side, gas-liquid flow becomes constant after only a few rows. Thus, the void fraction was only measured at one location. A single-beam, gamma-ray densitometer was used to measure void fractions near row 7 in the maximum gap between the rows. Corresponding pressure drops were obtained between rows 3 and 10. Data are presented for a mass flux range of 25–688 kg/m 2 s and a gas mass fraction range of 0.0005–0.6. The measurements are shown to compare reasonably well with predictions from correlations available in the open literature. This shows that these methods can be used for tube-bundles containing larger diameter tubes. Some elements of a heat-exchanger design require a more complex analysis. For example, tube vibration calculations require the distribution of void and phase velocity along the tube length. Such analysis can be provided by multiphase computational fluid dynamic (CFD) simulations. CFD approaches to modelling these flows require empirical inputs for the drag coefficient and the force on the fluid by the tubes. These are deduced from the measured data. The wall forces are shown to scale well with increased tube diameter, however, caution is required when selecting the drag coefficients.

We present an experimental evaluation of the peripheral finned-tube heat exchanger. In this novel compact evaporator geometry, the air-side is composed by an arrangement of open-pore cells formed by radial fins whose bases are attached to the tubes and whose tips are connected to peripheral fins. Each fin arrangement is made up of six radial fins and six peripheral fins forming a hexagon-like structure. The air-side fin configuration is composed of three levels of fin arrangement, each characterized by the length of radial fin and mounted with a 30° offset from its neighboring level. Experimental data on the air-side heat transfer and pressure drop were generated in an open-loop wind tunnel calorimeter. A one-dimensional theoretical model based on the theory of porous media has also been developed to predict the thermal-hydraulic behavior of the heat exchanger. The model incorporates the actual fin geometry into the calculation of the air-side porosity. The air-side permeability is calculated according to the Kozeny-Carman model with the particle diameter definition due to Whitaker and the friction factor correlation due to Ergun. The model overpredicts the air-side thermal conductance by less than 15% for air flow rates higher than 14 L/s. The air-side pressure drop is underpredicted by the model, but still within the limits encountered in the literature. The analysis is complemented with an entropy generation minimization analysis in order to demonstrate the procedure for obtaining an optimized configuration of the heat exchanger.