Heat transfer model of an iCVD reactor

Contrary to conventional HWCVD, the power consumption in the iCVD process is dominated by heat conduction rather than radiation. This is due to the fact that while the typical wire temperature for HWCVD is about 1750-2200 °C, for iCVD the temperature is only 250-500 °C. Typical deposition pressures are in the transition regime between the collision free regime, where the conduction is pressure dependent, and the collision mediated regime, where the conduction is pressure independent. The power loss due to heat conductivities of molecular nitrogen, glycidyl methacrylate (GMA) and tert-butylperoxide (TBPO) gases have been determined experimentally for these pressure regimes. The necessary power input to the filaments can be explained to be due to mainly heat dissipation by radiation and by gas conduction. This means that the dissociation process requires only very little power, about 2% of the total power consumption in a typical iCVD process.     

Ammonia-water desorption heat and mass transfer in microchannel devices

This paper presents the results of an experimentally validated model for the prediction of local heat and mass transfer rates in a microchannel ammonia-water desorber. The desorber is an extremely compact 178 mm × 178 mm × 0.508 m tall component capable of transferring the required heat load (∼17.5 kW) for a residential heat pump system. The model predicts temperature, concentration and mass flow rate profiles through the desorber, as well as the effective wetted area of the heat transfer surface. Previous experimental and analytical research by the authors demonstrated the performance of this same microchannel geometry as an absorber. Together, these studies show that this compact geometry is suitable for all components in an absorption heat pump, which would enable the increased use of absorption technology in the small-capacity heat pump market.     

Mathematical analysis for the efficiency of a semi-spherical fin with simultaneous heat and mass transfer

In this paper an analytical solution for the efficiency of a semi-spherical fin when subjected to simultaneous heat and mass transfer mechanisms is studied. For the mathematical analysis of a wet fin equation, a relationship between humidity ratio and temperature of the saturation air is needed. The driving forces for the heat and mass transfer are the temperature and humidity ratio differences, respectively. Analytical solutions are obtained for the temperature distribution over the fin surface when the fin is fully wet. It is observed that in humid conditions the fin has high efficiency to be used in industry. The variation effects of these parameters have been considered. Finally linear relation has been proposed for humidity and temperature on the fin surface.     

Analytical solution for the simultaneous heat and mass transfer problem in air washers

An analytical solution approach for the simultaneous heat and mass transfer problem in air washers operating as evaporative coolers is presented. A one-dimensional model using the coupled mass and energy balance equations in the air washer is presented. Then, starting from a linear approach for the experimental curve of the air saturation, an analytical solution for the model was derived. The solution showed an excellent agreement with the available results found in the literature. The influence of several important parameters for the cooling process such as temperature and ambient air humidity, air flow rate and feeding water temperature, in the air cooling rate was investigated. The efficacy of the process can be greatly increased by reducing the cooling water temperature and the applied air flow rate. The analytical solution can be easily included into the models used for simulating desiccant air-conditioning systems operating in conjunction with air washers.     

Heat and mass transfer processes in two phase He II/vapor

Modeling heat and mass transfer characteristics of two phase He II is discussed. The case considered assumes that the channel flow is one-dimensional and stratified, with mass exchange between the two phases. Two specific examples are considered in some detail. The first is the heat and mass transfer characteristics for small liquid flow rate. Use of several simplifying assumptions allows the problem to be reduced to solution of a one-dimensional ordinary differential equation. The result is a non-dimensional expression for the liquid level or void fraction along the channel. A set of dimensionless parameters are defined that establish the relative contributions of vapor mass transport and counterflow in the He II. The model also predicts the temperature profile and vapor mass flow rate. The second case concerns the flow of liquid under nearly isothermal conditions with relatively small vapor mass flow rate. Under these conditions, the flow may be modeled using classical hydrodynamics taking into consideration the unique characteristics of the He II. Results of these models are compared to experimental data for heat and mass transfer in a two phase He II/vapor flow.     

Heat and mass transfer characteristics in a spray chamber

This paper presents a one-dimensional mathematical model for heat and mass transfer of water droplets in a spray chamber. The model includes drop size distribution and velocity of the droplets generated by a nozzle of inlet diameter 3.2 mm. By using the conservation of mass and energy, the changes in water temperature, air temperature and humidity along the spray cone in the spray chamber can be calculated. This model is tested with two different water mass flows. The results look reasonable from practical point of view and they also show that higher water mass flow results in a higher air temperature drop and higher humidity.     

Study on nanoparticle production process characteristics in transferred arc plasma system using heat and mass balance

The nanoparticle production process in a transferred arc plasma system was studied. The plasma temperature, particle heating time, and particle residence time in plasma were calculated using heat and mass balance with a lumped capacitance method. We analyzed the nanoparticle production characteristics based on different operating conditions by comparing the particle vaporization time with the particle residence time in the plasma. The limit size for particle vaporization was derived. With higher plasma power, the nanoparticle production rate increased and the energy consumption rate decreased. It was confirmed that the energy consumption rate reaches an optimal point according to the plasma power. Experiments to determine the nanoparticle production rate according to plasma power were also conducted and the experimental data were compared with numerical values. The results show that the error rate between the numerical values and experimental data was approximately ±18%. Therefore, the developed model which was studied could be useful for designing nanoparticle production process using a transferred arc plasma system because of its simple approach.     

Heat and mass transfer on a cylinder surface in cross flow under supersaturated frosting conditions

In this paper a semi-empirical model describing heat and mass transfer on a cylinder surface in humid air cross flow under supersaturated frosting conditions is presented. The lack of psychrometric data in the supersaturated zone of the psychrometric chart has historically impeded the ability of researchers to accurately predict heat and mass transfer in supersaturated air. The work described in this paper has been partially made possible by developing a systematic procedure to compute the properties of supersaturated moist air, especially in the low temperature zone of the psychrometric chart. Development of such a capability will allow us to predict the amount of frost collected on a coil, the frost deposition and coil heat transfer rates, frost thickness and frost surface temperature, and other important coil frost parameters under supersaturated conditions.     

Heat and mass transfer in two-layer elastic composite subjected to thermal infrared radiation

In this paper coupled thermal, mechanical and diffusive processes in two-layer semitransparent composite subjected to thermal infrared radiation are examined. One of the layers is supposed to be multi-component consisting of deformable matrix and gaseous admixture. Effect of radiation upon a composite is taken into account by heat sources in composite and heat supply to admixture. Model equations are solved within iterative approach. On each step of iterative process radiation equations are solved analytically while for solving diffusion and thermal conductivity equations the finite difference method is applied. Numerical calculations suggested that the presence of the layer located closer to radiation source allows reduce thermal stresses in another layer of two-layer semitransparent elastic composite in both cases when materials of layers are the same and different. The layer located closer to radiation source and whose thickness is less than that of another layer is heated to lower temperatures, their distributions are more homogeneous and consequently, it undergoes weaker stress loading.     

Heat and mass transfer in an epitaxial reactor of the vertical type 1. Symmetrical heating of the reactor

Results are presented from a numerical study of the distribution of the dimensionless rate of deposition of an epitaxial layer along a reactor made in the form of a planar vertical convergent channel which is heated symmetrically.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 56, No. 6, pp. 942–948, June, 1989.     

Heat and mass transfer analysis in air-cooling of spherical food products

The one dimensional transient heat conduction equation, in spherical co-ordinates, is solved with convective surface boundary condition, during air-cooling. The enthalpy potential concept is used to include the cooling effect of desiccation. A calculation scheme is proposed in which, up to half the cooling time, the calculation is made with both heat and mass transfer from the product surface, thereafter it is continued with heat transfer only. The calculated temperatures for apples and potatoes are compared with the measured values available in the literature, and good agreement is observed.     

Heat transfer and pressure drop in a plate-type evaporator

A plate-type evaporator, working with natural refrigerant circulation, has been investigated both experimentally and theoretically. Motivated by the phase-out of ozone-depleting substances, HCFC22 was compared to HFC134a and two zeotropic refrigerant mixtures. The effect of different separator liquid levels, i.e. refrigerant flows, and its influence on heat transfer was also studied. The investigated plate-type evaporator consists of thirteen vertical flow channels and its size is 3.0 m × 0.5 m. The heat source for the evaporator is a falling water film on the outside of the plate. Experimental studies have been carried out using a test facility that enabled detailed measurements of heat transfer and pressure drop. Experiments were compared to results from a calculation method that simultaneously calculates heat transfer and pressure drop in a variable number of steps along the evaporator. The calculation method is based on a pressure drop correlation proposed by the VDI-Wärmeatlas and a heat transfer correlation for vertical tubes proposed by Steiner and Taborek. For different evaporator duties, heat transfer was over predicted by 12% for pure fluids by 15% for mixtures. Calculated pressure drops were well within ±5% of the measured values. Changes in heat transfer due to different flows were closely predicted by the proposed calculation method.     

Evaluation of heat and mass transfer coefficients for falling-films on tubular absorbers

A coupled heat and mass transfer model is developed to extract the transfer coefficients for falling-films from the measurements on a tubular absorber. The mass transfer coefficients obtained from the coupled model and the log-mean-difference approach agree within about 10%. For the heat transfer coefficient, the values given by the two models can differ quite significantly. The cooling water temperature distribution predicted by the coupled model agrees well with measurements. The transfer coefficients obtained from experimental measurements using the various methods reported in the literature show wide variations.     

CFD model of ITER CICC. Part VI: Heat and mass transfer between cable region and central channel

Dual-channel cable-in-conduit conductors (CICC) are used in the superconducting magnets for the International Thermonuclear Experimental Reactor (ITER). As the CICC axial/transverse size ratio is typically ∼1000, 1D axial models are customarily used for the CICC, but they require constitutive relations for the transverse fluxes. A novel approach, based on Computational Fluid Dynamics (CFD), was recently proposed by these authors to understand the complex transverse thermal-hydraulic processes in an ITER CICC from first principles. Multidimensional (2D, 3D) Reynolds-Averaged Navier-Stokes models implemented in the commercial CFD code FLUENT were validated against compact heat exchanger and ITER-relevant experimental data, and applied to compute the friction factor and the heat transfer coefficient in fully turbulent spiral rib-roughened pipes, mimicking the central channel of an ITER CICC. That analysis is extended here to the problem of heat and mass transfer through the perforated spiral separating the central channel from the cable bundle region, by combining the previously developed central channel model with a porous medium model for the cable region. The resulting 2D model is used to analyze several key features of the transport processes occurring between the two regions including the relation between transverse mass transfer and transverse pressure drop, the influence of transverse mass transfer on axial pressure drop, and the heat transfer coefficient between central channel and annular cable bundle region.     

Heat transfer and flow of He II in narrow channels

Heat transfer and fluid flow of He II in a long, narrow channel connected to a bath that supplies a constant supply of heat have been investigated by numerical simulations by using the simplified model of Kitamura et al. [Cryogenics 37 (1) (1997) 1]. Such channels are used to cool compact, stable, low-temperature magnets. The fluid flow is driven by natural convection and the mutual friction between the normal fluid and the superfluid.In this model, the thermomechanical effect and the Goter-Mellink mutual friction balance each other. A consequence of this balance is that the velocity and temperature distributions of He II can be characterized by a dimensionless, dependent parameter equal to the ratio of the fluid speeds of internal convection to the total fluid flow. After a sudden application of heat flux, the internal convection dominates over the total fluid flow until the establishment of steady-state temperature gradients. This predicts that the time required to set up the steady-state total fluid flow is proportional to the total heat capacity in the channel.     

Heat transfer in turbulent nanofluids: Separation flow studies and development of novel correlations

Convective heat transfer plays a significant role in numerous industrial cooling and heating applications. This method of heat transfer can be passively improved by reconfiguring flow passage, fluid thermophysical properties, or boundary conditions. The broader scope of nanotechnology introduced several studies of thermal engineering and heat transfer. Nano-fluids are one of such technology which can be thought of engineered colloidal fluids with nano-sized particles. In the present study, turbulent forced convection heat transfer to nanofluids in an axisymmetric abrupt expansion heat exchanger was investigated experimentally. During heat transfer investigation, the functionalized multiwalled carbon nanotubes (MWCNT-COOH), polycarboxylate functionalized graphene nanoplatelets (F-GNP), SiO₂ and ZnO water-based nanofluids were used. The convective heat transfer coefficient of fully developed turbulent flow of nanofluids flowing through an abrupt enlargement with the expansion ratio (ER) of 2 was experimentally determined at a constant wall heat flux of 12,128.56 W/m². The experiments were conducted at the Re ranges of 4000–16,000. The observed Nusselt numbers were higher than in the case of fully developed pipe flow indicating the level of the turbulent transport is high even though the recirculating velocities were a few percentages of the bulk mean velocity. The effect of Reynolds number and nanofluid’s volume concentration on heat transfer and friction losses were studied, where all the results reveal that with the increase of weight concentration and Reynolds number, the local Nusselt number enhanced at the increment of axial ratios in all the cases showing greater heat transfer rates than those of the base fluids. Comparison between the examined four types of nanofluids, show that the carbon-based nanofluids have a greater effect on enhancing heat transfer (33.7% and 16.7% heat transfer performance improvement for F-GNP and MWCNT nanofluids respectively at 0.1 wt% concentration) at the downstream of the sudden expansion pipe. There is no reported work dealing with the prediction of the local Nusselt number at the distance equivalent to the axial ratio and flow through sudden expansion. So far, two excellent correlations for the Local Nusselt number are proposed with reasonably good accuracy. Furthermore, a new correlation is developed for the average Nusselt number.     

Combined effects of external mass transfer and biodegradation rates on removal of phenol by immobilized Ralstonia eutropha in a packed bed reactor

Biodegradation of phenol by calcium-alginate immobilized Ralstonia eutropha was carried out in a batch stirred and a packed bed reactor. In the batch system studies, the effect of initial phenol concentration on biodegradation was investigated at 30 degrees C and pH 7 while in the continuous system studies, the effects of flow rate and inlet phenol concentration on biodegradation were tested at the same temperature and pH. The observed biodegradation rate constant was calculated at different flow rates with the assumption of first-order biodegradation kinetics. Various external mass transfer correlations were evaluated and a new correlation of the type JD=K(NRe)(-(n-1)) was developed with the values of K=1.34 and n=0.65. The intrinsic first-order biodegradation rate constants and the external mass transfer coefficients were calculated then the combined effects of these rates on the observed first-order biodegradation rate constants were also investigated.     

CFD simulation of coupled heat and mass transfer through porous foods during vacuum cooling process

A numerical simulation by using a computational fluid dynamics (CFD) code is carried out to predict heat and mass transfer during vacuum cooling of porous foods on the basis of mathematical models of unsteady heat and mass transfer. The simulations allow the simultaneous prediction of temperature distribution, weight loss and moisture content of the meats at low saturation pressure throughout the chilling process. The simulations are also capable of accounting for the effects of the dependent variables such as pressure, temperature, density and water content, thermal shrinkage, and anisotropy of the food. The model is verified by vacuum cooling of cooked meats with cylindrical shape within an experimental vacuum cooler. A data file for pressure history was created from the experimental pressure values, which were applied in the simulations as the boundary condition of the surface temperature.     

Coupled heat and mass transfer during nonisothermal absorption by falling droplet with internal circulation

We considered mass and heat transfer during nonisothermal absorption of a gas by a falling droplet with internal circulation. Gas phase is assumed to be free of inert admixtures and mass transfer is liquid phase controlled. Mass flux is directed from a gaseous phase to a droplet, and the interfacial shear stress causes a fluid flow inside the droplet. Droplet deformation under the influence of interface shear stress is neglected. Absorbate accumulation and temperature increase in the bulk of liquid phase are taken into account. The problem is solved in the approximations of a thin concentration and temperature boundary layers in the liquid phase. The thermodynamic parameters of the system are assumed constant. The system of transient partial parabolic differential equations of convective diffusion and energy balance with time-dependent boundary conditions is solved by combining the similarity transformation method with Duhamel's theorem, and the solution is obtained in a form of Volterra integral equation of the second kind which is solved numerically. Theoretical results are compared with available experimental data for water vapor absorption by falling droplets of aqueous solution of LiBr.     

Two-phase heat transfer and pressure drop of LNG during saturated flow boiling in a horizontal tube

Two-phase heat transfer and pressure drop of LNG (liquefied natural gas) have been measured in a horizontal smooth tube with an inner diameter of 8 mm. The experiments were conducted at inlet pressures from 0.3 to 0.7 MPa with a heat flux of 8–36 kW m⁻², and mass flux of 49.2–201.8 kg m⁻² s⁻¹. The effect of vapor quality, inlet pressure, heat flux and mass flux on the heat transfer characteristic are discussed. The comparisons of the experimental data with the predicted value by existing correlations are analyzed. Zou et al. (2010) correlation shows the best accuracy with 24.1% RMS deviation among them. Moreover four frictional pressure drop methods are also chosen to compare with the experimental database.