Analytic modelling of a falling film absorber and experimental determination of transfer coefficients

Accurate interpretation of the experimental data on falling film flows is a critical part of the investigations in the field of absorption energy system research. However, there is no theoretically proven way to determine experimental heat and mass transfer coefficients for non-isothermal absorption falling film flows. In this article, firstly, it is shown how the governing equations of a falling film absorber can be reduced to two ordinary differential equations and analytic expressions can be obtained for the temperature and concentration profiles along the absorber. Secondly, a new method is proposed to determine heat and mass transfer coefficients from experimental data and its application is demonstrated by reprocessing the experimental data from two experimental studies reported in the literature. The results show that some of the experimental data were misinterpreted by conventional methods and the errors were negligible only when heat and mass fluxes were small, which agrees with the fact that the obtained analytic solutions approach the conventional logarithmic heat and mass transfer equations in such conditions.     

Numerical heat transfer analysis of encapsulated ice thermal energy storage system with variable heat transfer coefficient in downstream

During charging and discharging processes, the heat transfer behavior of the encapsulated ice thermal energy storage (TES) system changes during downstream case and this should be taken into account since the temperature of heat transfer fluid (HTF) and especially the heat transfer coefficient varies considerably around each capsule. This requires a careful study of the problem with variable heat transfer coefficient to contribute to the state-of-the-art. This has been the primary motivation behind the present study. Here, we first develop a new heat transfer coefficient correlation by simulating a series of 120 numerical experiments for different capsule diameters, mass flow rates and temperatures of HTF and second undertake a comprehensive numerical analysis using the temperature based fixed grid solution with control volume approach for studying the heat transfer behavior of an encapsulated ice TES system. Thirdly, we validate the present numerical model and the new correlation with some experimental data obtained from the literature, and hence a good agreement is obtained between the model results and experimental data. The results indicate that the heat transfer coefficient varies greatly during downstream and highly affects the heat transfer taking place during the process. So, the solutions with constant heat transfer coefficient appear to be unreliable for analysis and system optimization. The results also show that the solidification process is chiefly governed by the magnitude of Stefan number, capsule diameter and capsule row number.     

Numerical study of heat and mass transfer of binary mixtures condensation in mini-channels

This work presents a model to study condensation heat and mass transfer characteristics of binary mixtures inside mini-channels. By considering the mass and heat transfer resistances in the vapor phase, the conservation equations of mass, species, momentum and energy are solved using higher order finite element method. The model is validated by comparing the predicted results with experimental data from the literature, in particular for the case of methane/ethane mixtures at different compositions and working conditions in a tube of 1.0 mm in diameter. The results show a reasonably good prediction of the heat transfer coefficient and pressure drop by comparing with these experimental data. The model is then used to study the effect of mass flux, wall heat flux and system inlet pressure on the heat and mass transfer resistances during condensation of binary mixtures.     

Fundamental Issues Related to Flow Boiling and Two-Phase Flow Patterns in Microchannels – Experimental Challenges and Opportunities

ABSTRACT

Flow boiling heat transfer in microchannels is used today in many diverse applications. The previous studies addressing the effect of channel size, heat flux, vapor quality, and mass flux on heat transfer during flow boiling are reviewed in the present paper. The relationship between flow characteristics and flow boiling heat transfer was studied experimentally for refrigerant R-C318 at moderate reduced pressures where the contribution of nucleate boiling is decisive. Flow boiling mechanisms were identified using an annular microchannel with transparent outer wall for successive visualization of boiling. The considerable suppression of nucleate boiling heat transfer was observed at transition to annular flow and explained by formation of a liquid flow with thin film and dry spots. A general equation for prediction of two-phase flow boiling heat transfer inside the circular, annular, and rectangular microchannels is proposed and verified using the experimental data. This equation accounts for the nucleate boiling suppression, forced convection, and thin film evaporative heat transfer in the form that allows to distinguish more clearly the contribution of each mechanism of heat transfer under the conditions, when it is predominant. A new approach for prediction of transition to the annular flow is proposed and verified, using the experimental data.     

Numerical and experimental analysis of moisture transfer for convective drying of some products

Numerical and experimental results of moisture transfer in drying process for apple and potato slices are compared in this study. Experimental results are obtained using a cyclone type dryer. Two-dimensional analysis of heat and moisture transfer during drying of objects is carried out solving heat and mass equations using finite-volume approach. Thus, moisture distributions inside the moist objects are obtained at different time steps. Comparison of results showed that there is a considerably high agreement between experimentally measured data and predicted values. Moist distribution also presented inside the products at different time periods.     

3d-Transient modeling of heat and mass transfer during heat treatment of wood

In the current work, three-dimensional Navier-Stokes equations along with the energy and concentration equations for the fluid coupled with the energy and mass conservation equations for the solid (wood) are solved to study the transient heat and mass transfer during high thermal treatment of wood. The model for wood is based on Luikov's approach and solves a set of coupled heat and mass transfer equations. The model equations are solved numerically by the commercial package FEMLAB for the temperature and moisture content histories under different treatment conditions. The simulation of the proposed conjugate problem allows the assessment of the effect of the heat and mass transfer within wood on the transfer in the adjacent gas, providing good insight on the complexity of the transfer mechanisms.     

Integrated analysis of cooling water systems: Modeling and experimental validation

Cooling towers are widely used in many industrial and utility plants as a cooling medium, whose thermal performance is of vital importance. Despite the wide interest in cooling tower design, rating and its importance in energy conservation, there are few investigations concerning the integrated analysis of cooling systems. This work presents an approach for the systemic performance analysis of a cooling water system. The approach combines experimental design with mathematical modeling. An experimental investigation was carried out to characterize the mass transfer in the packing of the cooling tower as a function of the liquid and gas flow rates, whose results were within the range of the measurement accuracy. Then, an integrated model was developed that relies on the mass and heat transfer of the cooling tower, as well as on the hydraulic and thermal interactions with a heat exchanger network. The integrated model for the cooling water system was simulated and the temperature results agree with the experimental data of the real operation of the pilot plant. A case study illustrates the interaction in the system and the need for a systemic analysis of cooling water system. The proposed mathematical and experimental analysis should be useful for performance analysis of real-world cooling water systems.     

Numerical investigation of simultaneous heat and mass transfer mechanisms occurring in a gypsum board exposed to fire conditions

This paper investigates the simultaneous heat and mass transfer mechanisms occurring in a gypsum board exposed to fire conditions. An in-house developed code (HETRAN), simulating heat and mass transfer in porous building materials, has been used to predict the heat and mass transfer characteristics within gypsum boards. The code solves numerically a set of mass and energy equations appropriate for the heat and mass transfer in porous materials, assuming homogeneity, local thermodynamic equilibrium and mass transfer due to diffusion and pressure gradients. The predicted temperature evolution within the gypsum sample, with and without mass transfer, is compared to experimental data, demonstrating that vapor migration through the sample holds a significant role in the board behavior under elevated temperatures. The results demonstrate that vapor migrates towards both directions of the board (fire and ambient side), with diffusion mass transfer being the dominant mass transfer mechanism, whereas air moves towards the “fire side”. The dehydration front moves from the “fire side” to the ambient side, with a high velocity in the beginning, which reduces as the front moves through the gypsum sample to the ambient side.     

A modified k–ε turbulence model for the simulation of two-phase flow and heat transfer in condensers

A modified k–ε turbulence model is developed in this study to simulate the gas–liquid two-phase flow and heat transfer in steam surface condensers. A quasi-three-dimensional algorithm is used to simulate the fluid flow and heat transfer in steam surface condensers. The numerical method is based on the conservation equations of mass and momentum for both gas-phase and liquid-phase, and mass fraction conservation equation for non-condensable gases. The numerical simulation of an experimental steam surface condenser has been conducted using the proposed modified k–ε turbulence model. The results obtained from the proposed model agree well with the experimental results and the results also show an obvious improvement in the prediction accuracy comparing with previous results where a constant value for the turbulent viscosity was used.     

Experimental study on heat and mass transfer characteristics of louvered fin-tube heat exchangers under wet condition

An experimental study was conducted to investigate the effect of a tube row, a fin pitch and an inlet humidity on air-side heat and mass transfer performance of louvered fin-tube heat exchangers under wet condition. Experimental conditions were varied by three fin pitches, two rows, and two inlet relative humidities. From the experimental results, it was found that the heat transfer performance decreased and the friction increased with the decrease of a fin pitch, for 2 row heat exchanger. The effect of a fin pitch on heat transfer performance was negligible with 3 row heat exchanger. The change in a relative humidity was not affected heat transfer and friction. However, the mass transfer performance was slightly decreased with the increase of a relative humidity and with the decrease of a fin pitch. The mass transfer performance decreased with the decrease of a fin pitch. The mass transfer performance of the louvered fin-and-tube heat exchanger was different according to the number of a tube row.     

Experimental study on condensation heat transfer enhancement and pressure drop penalty factors in four microfin tubes

Heat transfer and pressure drop characteristics of four microfin tubes were experimentally investigated for condensation of refrigerants R134a, R22, and R410A in four different test sections. The microfin tubes examined during this study consisted of 8.92, 6.46, 5.1, and 4 mm maximum inside diameter. The effect of mass flux, vapor quality, and refrigerants on condensation was investigated in terms of the heat transfer enhancement factor and the pressure drop penalty factor. The pressure drop penalty factor and the heat transfer enhancement factor showed a similar tendency for each tube at given vapor quality and mass flux. Based on the experimental data and the heat-momentum analogy, correlations for the condensation heat transfer coefficients in an annular flow regime and the frictional pressure drops are proposed.     

Experimental investigation of thermal and moisture behaviors of wet and dry soils with buried capillary heating system

An experimental study of thermal and moisture behaviors of dry and wet soils heated by buried capillary plaits was done. This study was carried out on a prototype similar to an agricultural tunnel greenhouse. The experimental procedure consisted on three different measuring phases distinguished by three different operational conditions of the capillary plaits: heating at 70 °C, heating at 40 °C and without heating in summer. During an experimental run, quantities measured are soil temperature, soil water content at various depths, soil surface heat flux, solar radiation under the plastic cover, internal relative humidity, internal and external air temperature. In unsaturated moist soils, the transport of heat is complicated by the fact that heat and mass transfer is a coupled process. During the daily soil temperature variation, it was found that the surface temperature amplitude was higher in wet soil than in dry soil. The water content increased during daytime and decreased during nighttime. The diurnal variation amplitude of water content was higher without underground heating and decreased with the buried heat source temperature.     

Comparative investigations of the condensation of R134a and R404A refrigerants in pipe minichannels

The present paper describes the results of experimental investigations of heat transfer and pressure drop during the condensation of the R134a and R404A refrigerants in pipe minichannels with internal diameters d = 0.31–3.30 mm. The results concern investigations of the local heat transfer coefficient and a pressure drop in single mini-channels. The results were compared with calculations according to the correlations proposed by other authors. Within the range of the examined parameters of the condensation process in mini-channels produced from stainless steel, it was established that the values of the heat transfer coefficient may be described with Akers et al. and Shah correlations within a limited range of the mass flux density of the refrigerant and the mini-channel diameter. A pressure drop during the condensation of these refrigerants is described in a satisfactory manner with Friedel and Garimella correlations. On the basis of the experimental investigations, the authors proposed their own correlation for the calculation of local heat transfer coefficient α_x.     

Finite-volume modelling of heat and mass transfer during convective drying of porous bodies – Non-conjugate and conjugate formulations involving the aerodynamic effects

In this study, a numerical procedure is outlined and representative results for heat and mass transfer during convective drying of porous bodies are presented. The Luikov model was implemented and applied both on individual samples of construction materials and agricultural products, as well as on a drying-chamber scale, with parallel flow of a hot air stream over rectangular slabs which represent the product to be dried. In the latter case the configuration is an experimental dryer in which the heat source is a solar air collector with evacuated tubes. A general approach was developed that allows a selection between modelling of phenomena either in the drying solid only, or considering an extended simulation domain encompassing, apart from the solid body, the flow of air as well. In the second case, the solution of the flow field is pursued along with a conjugate heat/mass transfer problem coupling the solid and fluid phenomena and in both cases phase change (evaporation) was taken into account. For the numerical simulation, the finite-volume method was used. The validation of the model was based on experimental and numerical results from the literature and results from simulations that were conducted in the pursuit of the energetic optimization of an experimental solar dryer of NCSR “Demokritos” are presented. In the latter case, the effect of the particular flow field features developing for a single and a double-plate configuration on the heat/mass transport and drying rates is demonstrated. Such a methodology could be used to analyze the transport phenomena in any type of convective dryer, including those utilizing solar energy as the heat source.     

Design and Investigation of Heat Transfer in a New Adsorbent Bed With CPC for Solar Adsorption Refrigeration Systems

This article presents the design and the heat transfer study in a novel adsorbent bed with compound parabolic concentrator (CPC) for solar adsorption chillers. The objectives of the study were to investigate the heat transfer in the adsorbent bed experimentally, and to verify the fins layout through finite-element analysis (FEA) simulation. CPCs with different concentration ratios were experimentally tested and an appropriate design of CPC was selected for a prototype. The prototype was designed with the objective of improving the heat and mass transfer ability of the adsorbent bed. Fins were placed in the transverse direction under the receiver area of each CPC. Spaces were provided from three sides of the adsorbent for easy movement of the refrigerant. FEA software was used to study the effect of the fins layout and fins pitch. The experimental results showed that the heat was efficiently transferring up to the end and extended parts of the bed. Simulation results indicated that the present strategy of placing the fins in a transverse direction gives uniform heat distribution compared to a fins layout with fins placed in a longitudinal direction. The proposed design scheme will be helpful to improve the system performance by increasing the heat and mass transfer ability of an adsorbent bed.     

Prediction of two‐phase condensation characteristics of some alternatives to R‐22 inside air/refrigerant enhanced surface tubing

The results of an experimental study on the heat transfer characteristics of two‐phase flow condensation of some azeotropic refrigerant mixtures, proposed as alternatives to R‐22, on air/refrigerant horizontal enhanced surface tubing are presented. The condensation data indicated that the heat transfer coefficient of the blend R‐408A has the highest heat transfer rate among the blends under investigation. The condensation data also showed that R‐507 and R‐404A have similar heat transfer rates to that of R‐22 when plotted against the refrigerant mass flow rate. It can also be observed that, as the mass flux increases, the heat transfer coefficient increases. Correlations were proposed to predict the heat transfer characteristics such as average heat transfer coefficients as well as pressure drops of alternatives to R‐22 such as R‐507, R‐404A, R‐407C and R‐408A, as well as R‐410A in two‐phase flow condensation inside enhanced surface tubing. In addition, proposed correlations were found to fairly predict the two‐phase flow heat transfer condensation data. Copyright © 2000 John Wiley & Sons, Ltd.     

New proposed two-phase multiplier and evaporation heat transfer coefficient correlations for R134a flowing at low mass flux in a multiport minichannel

New correlations of the two-phase multiplier and heat transfer coefficient of R134a during evaporation in a multiport minichannel at low mass flux are proposed. The experimental results were obtained from a test using a counter-flow tube-in-tube heat exchanger with refrigerant flowing in the inner tube and hot water in the gap between the outer and inner tubes. Test section is composed of the extruded multiport aluminium inner tube with an internal hydraulic diameter of 1.2 mm and an acrylic outer tube with an internal hydraulic diameter of 25.4 mm. The experiments were performed at heat fluxes between 10 and 35 kW/m², and a refrigerant mass flux between 45 and 155 kg/(m² s). Some physical parameters that influenced the frictional pressure drop and heat transfer coefficient are examined and discussed in detail. The pressure drop and heat transfer coefficient results are also compared with existing correlations. Finally, new correlations for predicting the frictional pressure drop and heat transfer coefficient at low mass fluxes are proposed.     

Heat exchanger design considering variable overall heat transfer coefficient: An artificial neural network approach

This study presents an artificial neural network approach in combination with numerical methods to calculate the heat transfer area assuming a nonlinear variation of the global heat transfer coefficient as a consequence of the thermophysical properties of the fluids, the geometry of the surfaces, and other factors. The development of the article is presented in two applications. The first application takes up the database described by Allan P. Colburn, four possibilities are proposed using functions from the field of artificial neural networks to create several approaches. The second application is presented to verify the goodness of the proposed methodology, the artificial neural network model is applied in an experimental data set of double-pipe vertical heat exchangers, the comparison between the calculated and experimental heat transfer area shows a relative percentage error smaller than 2.8%. The results in the applications are evidence of the competitiveness of the artificial neural network for the prediction of the heat transfer area considering a variable overall heat transfer coefficient.     

Heat transfer during air flow in aluminum foams

This paper presents experimental heat transfer coefficients measured during air flow heating in seven different aluminum open-cell foam samples with different number of pores per inch (PPI), porosity and foam core height under a wide range of air mass velocity. Three imposed heat fluxes are considered for each foam sample: 25.0, 32.5 and 40.0 kW m^?2. The collected heat transfer data are analyzed to obtain the global heat transfer coefficient and the normalized mean wall temperature. A model from the open literature has been selected and compared with the experimental database. A new simple heat transfer model, based on the present data, for the global heat transfer coefficient and the foam-finned surface area efficiency estimations has then been developed and compared against the experimental measurements.