A new method for determining coupled heat and mass transfer coefficients between air and liquid desiccant

This paper presented the characteristic of liquid desiccant dehumidification based on NTU–Le model. The results showed that the Lewis number Le had little effect on air outlet humidity ratio during desiccant solution dehumidification process. A new method called h_D–Le separative evaluation method was developed for determining coupled heat and mass transfer coefficients between air and liquid desiccant, through which the heat and mass transfer coefficients between air and liquid desiccant were calculated to obtain from experimental inlet and outlet parameters of air and desiccant solution. The effects of the air volume flow rate, temperature, humidity ratio and the solution concentration, temperature on the Lewis number, heat and mass transfer coefficient were analyzed according to experimental data and the h_D–Le separative evaluation method. Based on the computation results, it was concluded that the Lewis number greatly depended on the operation parameters and conditions of the air and desiccant. In addition, the correlations of the heat and mass transfer coefficients were developed. The additional 74 groups of experiments validated the developed correlations by comparison of air/solution parameters change with the calculation data.     

Modeling and performance analysis of solar air pretreatment collector/regenerator using liquid desiccant

A solar liquid regenerator that embodies energy saving effect is a key part in solar liquid cooling air-conditioning system. Solar air pretreatment liquid collector/regenerator as a novel solar C/R (collector/regenerator) can achieve liquid regeneration in lower temperature, which is suitable to be employed in the high humidity area. The heat and mass transfer process was simulated in the novel liquid regenerator and the conclusions show that the increment of solution outlet concentration increases 70%, regeneration efficiency η_z augments 45.7% and storage capacity SC increases 44% as effective solution proportion ESP falls from 100% to 62%. For higher solution outlet concentration needed in the dehumidifier, both lower solution mass flow rate and higher solution inlet concentration all can be adopted in the novel C/R, in which the decrease of effective solution proportion ESP can increase the rate of evaporation G significantly. Along with the augment of air mass flow rate, the rate of evaporation G rises fast firstly and then falls slowly. The simulated results show that there is huge potential of improving and regulating solution regeneration performance by employing the novel C/R.     

SOLAR ASSISTED OPEN-CYCLE ABSORPTION COOLING: PERFORMANCE OF COLLECTOR/REGENERATORS

In this study, a side-by-side test was performed on a glazed and an unglazed collector/regenerator operating under identical environmental conditions. This test procedure differed from previous experiments in that the inlet solution state was maintained constant during the period of testing. Also, for the glazed C/R, local solution film temperatures as well as entrance and exit air dry and wet bulb temperatures were measured. With the use of experimental data, empirical correlations were developed for heat and mass transfer coefficients in terms of Nusselt and Sherwood numbers. These correlations were used in the simulation study to identify important variables affecting the performance of each collector/regenerator. The performance of the glazed collector/regenerator was considerably affected by solution flow rate, solar irradiation, ambient temperature, solution temperature and concentration at the inlet, and glazing height. The evaporation rate for the unglazed collector/regenerator was strongly dependent upon ambient temperature, humidity, wind speed, and solution concentration at the inlet to the collector/regenerator. Generally, it was found that the unglazed C/R performed better than the glazed C/R for the conditions considered in this study. Contrary to previous research, this data showed an increase in evaporation rate as the gap height was decreased from 15 to 7 cm. The glazing, also helped to maintain cleaner absorbent solution and reduced waste due to rain. © 1997 by John Wiley & Sons, Ltd.     

Simulation of airflow in one- and two-room enclosures containing a fire source

In this study, the airflow in a room that contains a heat source is simulated numerically. The flow is considered turbulent and buoyant. The results of the mathematical model are validated with available experimental data at specific locations in the domain. A simple geometry is adopted, consisting of a room with a door that plays the role of both inlet–outlet for the fluid (air). At the centre of the room a methane burner is placed to serve as a heat source. The problem is simulated using two turbulence models, the well-known standard k–ε model and the RNG k–ε model, both modified to account for buoyancy effects on turbulence. The burner is considered as a volumetric heat source. It is concluded that the fire plume development as well as the distributions of velocity and temperature are reasonably well predicted. Following this conclusion, both models are also applied to a different, more complex geometry that consisted of two rooms communicating via a door, while the heat source was placed in the first room. Unfortunately, there are no experimental data to compare with for this case, but the results appear plausible. Finally, important design factors, such as mass flow rates and neutral-plane heights, are calculated utilizing the CFD results, and are compared with those obtained by well-known empirical correlations. It is concluded that the bi-directional flow existing through the burning-room vent is similarly predicted by both turbulence models; the RNG k–ε model leading to higher, and more accurate predictions of temperature variations within the hot upper layer, at least for the single-room case.     

Optimal monolithic configuration for heat integrated ethanol steam reformer

A design concept for optimal design of monolith catalyst is presented through modeling of transport–kinetic interactions in a monolith catalyst. We argue that reactors employing monolithic catalysts should be based on its optimal choice of geometry. In line with that argument, we present a thorough analysis of the geometrical parameters influencing the performance of non-isothermal reactor operation. In this study, an optimal monolith configuration is estimated to be a combination (d_h, t_w) of (0.9 mm, 0.2 mm) for a compact ethanol reformer to produce hydrogen for portable applications where maximum volumetric reactor activity exists. A three-dimensional modeling framework is developed for the resulting optimal monolithic catalyst design that couples the reforming section with a suitable heat source in a recuperative way. As a result, greater ethanol conversion is obtained from the monolith channels near the periphery of the block. The coupling with combustion could predict the formation of cold and hot spots inside the reactor, their nature being dependent on the flow configuration. Further, the effect of altering the feed inlet operating conditions over the variation of ethanol conversion and temperature inside the reactor is also analyzed. The increase in reforming inlet velocity decreases the outlet conversion and shifts the cold spot, forward and deeper in co-flow configuration. The decreasing inlet feed temperature enhances the transfer of heat, eliminating the cold spot.     

Coupled heat and mass transfer characteristic in packed bed dehumidifier/regenerator using liquid desiccant

The dehumidifier and regenerator are two key components in liquid desiccant air conditioning systems. The heat transfer driving force and the mass transfer driving force influence each other, the air and desiccant outlet temperatures or humidity ratio may exceed the air and desiccant inlet parameters in the dehumidifier/regenerator. The uncoupled heat and mass transfer driving forces, enthalpy difference and relative humidity difference between the air and desiccant are derived based on the available heat and mass transfer model and validated by the experimental and numerical results. The air outlet parameter reachable region is composed of the air inlet isenthalpic line, the desiccant inlet equivalent relative humidity line and the linkage of the air and desiccant inlet statuses. Except the mass flow rate ratio and the heat and mass transfer coefficients, the air and desiccant inlet statuses and flow pattern have great effects on the dehumidifier/regenerator performance. The counter flow configuration expresses the best mass transfer performance in the dehumidifier and the hot desiccant driven regenerator, while the parallel flow configuration performs best in the hot air driven regenerator.     

Unglazed collector/regenerator performance for a solar assisted open cycle absorption cooling system

This study includes the results of an investigation on a prototype solar assisted absorption cooling system. The liquid absorbents, for example, lithium chloride or lithium bromide used in this type of system absorb water, the refrigerant, in the absorber and require energy input for a subsequent desorption process. An ordinary black shingled roof was used as a collector/regenerator for the evaporation of water to obtain a strong solution of absorbent for use in the absorber. The rate of evaporation from the collector/regenerator determines the overall cooling capacity of the system. Experiments were conducted on a 11 m × 11 m (36 ft × 36 ft) collector/regenerator to measure solution flow rates and concentrations at inlet and outlet, and temperatures at several locations of the collector. Altogether, there were 100 sets of data taken under various environmental and flow conditions. An iterative solution of the equations describing the conservation of mass and energy led to the prediction of local concentrations of the absorbent solution, and local heat and mass transfer coefficients. Correlations for nondimensional heat and mass transfer parameters were developed in terms of local Reynolds number, Grashof number, Prandtl number, Schmidt number, and N, the ratio of buoyancy force due to mass transfer to buoyancy force due to heat transfer. These heat and mass transfer correlations were used for the simulation of performance of the collector/regenerator. A parametric study of the collector/regenerator performance enabled the identification of important variables. A good agreement between these results and those from a warm, humid climate indicates that the simulation model can be used as a tool for the design of systems operating under similar conditions. The experimental results also show a regeneration efficiency varying between 38 and 67%, and the corresponding cooling capacities ranged from 31 to 72 kW (8.8 to 20 tons).     

Dimensional analysis for the heat transfer characteristics in the corrugated channels of plate heat exchangers

Using the Buckingham Pi theorem, this study derives dimensionless correlations to characterize the heat transfer performance of the corrugated channel in a plate heat exchanger. The experimental data are substituted into these correlations to identify the flow characteristics and channel geometry parameters with the most significant influence on the heat transfer performance. Simplified correlations by omitting the factors with less influence are then obtained. The results show that Nu_x is affected primarily by Re, R/D_h, x/D_h, and β. Neglecting the minor effect of factors on Nu_x, it is shown that Nu_m is determined primarily by Re, R/D_h and β.     

CFD simulation of a Gifford–McMahon type pulse tube refrigerator

Pulse tube refrigerator has the advantages of long life and low vibration over the conventional cryocoolers, such as Gifford–McMahon (GM) and Stirling coolers because of the absence of moving parts in low temperature. This paper performs a two-dimensional computational fluid dynamic (CFD) simulation of a Gifford–McMahon type double inlet pulse tube refrigerator (DIPTR), operating under a variety of thermal boundary conditions. A commercial Computational Fluid Dynamics (CFD) software package Fluent 6.1 is used to model the oscillating flow inside a pulse tube refrigerator. Helium is used as working fluid for the entire simulation. The simulated DIPTR consists of a transfer line, an after cooler, a regenerator, a pulse tube, a pair of heat exchangers for cold and hot end, an orifice valve with connecting pipe, a double inlet valve with connecting pipe and a reservoir. The simulation represents fully coupled systems operating in steady-periodic mode. The externally imposed boundary condition is sinusoidal pressure inlet by user defined function at one end of the tube and constant temperature or heat flux boundaries at the external walls of the hot end and cold-end heat exchangers. The general results, such as the cool down behaviors of the system, phase relation between mass flow rate and pressure at pulse tube section and the temperature profile along the wall of the cooler are presented.The simulation shows the minimum decrease in temperature at cold-end heat exchanger for a particular combination of cryocooler assembly. The CFD simulation results are compared with available experimental data. Comparisons show that there is a reasonable agreement between CFD simulation and experimental results.     

Explicit Design of Balanced Regenerators

An explicit procedure for the design and sizing of balanced regenerators has been developed. A set of performance curves relating the thermal ratio, harmonic mean reduced length and period, and minimum cold fluid outlet temperature is presented. The specific characteristics of the regenerator's matrix including the heat transfer and pressure drop correlations are used to develop interrelations, represented by a set of three design curves, between these quantitites and the operating characteristics of the regenerator. The performance and design curves are used in the design procedure to determine the dimensions of the regenerator for the specified operating conditions.     

Comparison of different current collecting modes of anode supported micro-tubular SOFC through mathematical modeling

A two-dimensional model comprising fuel channel, anode, cathode and electrolyte layers for anode-supported micro-tubular solid oxide fuel cell (SOFC), in which momentum, mass and charge transport are considered, has been developed. By using the model, tubular cells operating under three different modes of current collection, including inlet current collector (IC), outlet current collector (OC) and both inlet and outlet collector (BC), are proposed and simulated. The transport phenomena inside the cell, including gas flow behavior, species concentration, overpotential, current density and current path, are analyzed and discussed. The results depict that the model can well simulate the diagonal current path in the anode. The current collecting efficiency as a function of tube length is obtained. Among the three proposed modes, the BC mode is the most effective mode for a micro-tubular SOFC, and the IC mode generates the largest current density variation at z-direction.     

Airside characteristics of heat,mass transfer and pressure drop for heat exchangers of tube-in hydrophilic coating wavy fin under dehumidifying conditions

The airside heat, mass and momentum transfer characteristics of seven wavy fin-and-tube heat exchangers with hydrophilic coating under dehumidifying conditions were experimented. The test inlet air dry bulb temperatures were 20, 27 and 35 ^oC, the inlet relative humidity were 50%, 60%, 70% and 80%, and the air velocity were 0.5, 1.0, 2.0, 3.0 and 4.0 m s^?1. The test results indicate that both the Colburn j_m factor and the Colburn j_h factor decrease with the increase of fin pitch, and this phenomenon becomes more and more pronounced as Reynolds number decreases. The friction factor is very sensitive to the change of fin pitch, and the friction factor shows a cross-over phenomenon as fin pitch changes. The Colburn j_h factor decreases and the Colburn j_m factor increases when the number of tube rows increases, while the friction performance is insensitive to the change of the number of tube rows. The effects of inlet relative humidity on the heat transfer and friction performance can be omitted, but the Colburn j_m factor decreases with the increase of the inlet relative humidity. The predictive ability of the available state-of-the-art heat transfer and pressure drop correlations was evaluated with the experiment data of the present study. The new heat, mass and momentum transfer correlations were proposed to describe the present test results according to the multiple linear regression technique. The mean deviations of the proposed j_h, j_m and f correlations are 6.3%, 8.9% and 7.9%, respectively. Comparing to published data reduction method, the process line on psychrometric chart of fin-and-tube heat exchanger for partially wet conditions and more accurate overall heat transfer coefficient equation are put forward in this paper.     

Convective mass transfer coefficient for a hydrodynamically developed airflow in a short rectangular duct

In this paper, experiments are performed to determine the convective mass transfer coefficient for evaporation in a horizontal rectangular duct with an aspect ratio of 14.5:1. In the test facility, a short pan of water forms the lower panel of a long duct where a hydrodynamically fully developed laminar or turbulent airflow passes over the surface of the water. The measured convective mass transfer coefficients have uncertainties that are typically less than ±10% and are presented for Reynolds numbers (Re) between 570 and 8100, Rayleigh numbers (Ra) between 6300 and 83,000, inverse Graetz numbers (Gz) between 0.003 and 0.04, and operating conditions factors (H¹) between −3.6 and −1.4. The measured convective mass transfer coefficients are found to increase as Re, Ra, Gz and H¹ increase and these effects are included in the Sherwood number correlations presented in this paper, which summarize the experimental data.     

Shell-and-tube evaporator model performance with different two-phase flow heat transfer correlations. Experimental analysis using R134a and R1234yf

This work presents a model of a shell-and-tube evaporator using R1234yf and R134a as working fluids. The model uses the effectiveness-NTU method to predict the evaporation pressure and the refrigerant and secondary fluid temperatures at the evaporator outlet, using as inputs the geometry of the evaporator, the refrigerant mass flow rate and evaporator inlet enthalpy, and the secondary fluid volumetric flow rate and evaporator inlet temperature. The model performance is evaluated using different two-phase flow heat transfer correlations through model outputs, comparing predicted and experimental data. The output parameter with maximum deviations between the predicted and experimental data is the evaporating pressure, being the deviations in outlet temperatures less than 3%. The evaporator model using Kandlikar's correlation obtains the highest precision and the lowest absolute mean error, with 4.87% in the evaporating pressure, 0.45% in the refrigerant outlet temperature and 0.03% in the secondary fluid outlet temperature.     

Critical heat flux performance for flow boiling of R-134a in vertical uniformly heated smooth tube and rifled tubes

In the present paper, critical heat flux (CHF) experiments for flow boiling of R-134a were performed to investigate the CHF characteristics of four-head and six-head rifled tubes in comparison with a smooth tube. Both of rifled tubes having different head geometry have the maximum inner diameter of 17.04 mm while the smooth tube has the average inner diameter of 17.04 mm. The experiments were conducted for the vertical orientation under outlet pressures of 13, 16.5, and 23.9 bar, mass fluxes of 285-1300 kg/m²s and inlet subcooling temperatures of 5-40 °C in the R-134a CHF test loop. The parametric trends of CHF for the tubes show a good agreement with previous understanding. In particular, CHF data of the smooth tube for R-134a were compared with well-known CHF correlations such as Bowring and Katto correlations. The CHF in the rifled tube was enhanced to 40-60% for the CHF in the smooth tube with depending on the rifled geometry and flow parameters such as pressure and mass flux. In relation to the enhancement mechanism, the relative vapor velocity is used to explain the characteristics of the CHF performance in the rifled tube.     

Heat transfer coefficient and thermal losses of solar collector and Nusselt number correlation for rectangular solar air heater duct with longitudinal fins hold under the absorber plate

An experimental investigation has been carried out for a series of system and operating parameters in order to analyze the effect of mass flow rate on heat transfer and Nusselt number characteristics in solar air heater. Experiments are performed at different air mass flow rates; varying from 0.012 to 0.016 kg/s, about hot summer days of Mai 2012. Hourly values of global solar radiation and some meteorological data (temperature, wind speed, relative humidities, etc.) for measuring days are obtained from the Biskra city of Algeria. The experiments encompassed the flow Reynolds number in the range 965.48–1301.4. Longitudinal fins were used inferior the absorber plate for an increase the heat exchange and render the flow fluid in the channel uniform. The effects of mass flow rate of air on the outlet temperature, Nusselt Number, Reynolds Number, Prandtl Number, the heat transfer in the thickness and length of the solar air collector were studied. For this effect was have created a new correlation correspondent of solar air collector with using fins it was written Nu = κ₀Re^1.36Pr^?0.68exp(0.342m)h ^[?0.018Pr].     

Simultaneous heat and mass transfer characteristics for wavy fin-and-tube heat exchangers under dehumidifying conditions

The present study proposes a new reduction method to calculate the heat and mass transfer characteristics of the wavy fin-and-tube heat exchangers under dehumidifying conditions. For fully wet conditions, the sensible heat transfer and mass transfer characteristics are relatively insensitive to the inlet relative humidity. The heat and mass transfer performances show appreciable influence of fin spacing at 1-row configuration. Both the heat and mass transfer performances increase when the fin spacing is reduced. However, the difference becomes less noticeable when Re_Dc > 3000. For 1-row configuration, larger wave height shows much larger difference with the fin spacing. However, the effect of inlet conditions and geometrical parameters on the heat and mass performance becomes less significant with the rise of number of tube rows. Test results show that the heat and mass transfer analogy is roughly applicable (the ratios of h_c,o/h_d,oC_p,a are in the range 0.6–1.1, and is insensitive to change of fin spacing). The correlations are proposed to describe the heat and mass transfer characteristics. These correlations can describe 94.19% of the j_h factors within 15% and 83.72% of the j_m factors within 15%. Correspondingly, 93.02% of the ratios of h_c,o/h_d,oC_p,a are predicted by the proposed correlation within 15%.     

Flow maldistribution and thermal performance deterioration in a cross-flow air to air heat exchanger with plate-fin cores

The inlet and outlet duct geometry in an air to air compact heat exchanger is always irregular. A skewed Z-type arrangement is popular between the impinging flow and the core. Such duct placements usually lead to a non-uniform flow distribution on core surface. In this research, the flow maldistribution and thermal performance deterioration in cross-flow air to air heat exchangers are investigated. The inlet duct, the core and the outlet duct are combined together to calculate the flow distribution on core inlet face. First, a CFD code is used to calculate the flow distribution, by treating the plate-fin core as a porous media. Then a heat transfer model between the two air flows in the plate-fin channels is set up. Using the flow distribution data predicted, the heat exchange effectiveness and the thermal performance deterioration factor are calculated with finite difference scheme. Experiments are performed to validate the flow distribution and heat transfer model. The results indicate that when the channel pitch is below 2.0 mm, the flow distribution is quite homogeneous and the thermal deterioration due to flow maldistribution can be neglected. However, when the channel pitch is larger than 2 mm, the maldistribution is quite large and a 10–20% thermal deterioration factor could be found. The study proves that the inlet duct, the outlet duct, and the core should be coupled together to clarify flow maldistribution problems.     

Optimal geometry of L and C-shaped channels for maximum heat transfer rate in natural convection

The present paper documents the geometric optimization of L and C-shaped channels in laminar natural convection subject to global constraints. The objective is to maximize the heat transfer rate from the hot wall to the coolant fluid. Three different configurations were considered: (i) an L-shaped asymmetric vertical heated channel with an adiabatic horizontal inlet, (ii) an asymmetric vertical heated channel with an adiabatic vertical outlet, and finally, (iii) a C-shaped vertical channel with horizontal inlet and outlet. The two first configurations are free to morph according to two degrees of freedom: the wall-to-wall spacing and inlet (or outlet) height. The third configuration is optimized with respect to the wall-to-wall spacing, and the heights of the inlet and outlet ports. The effect of the inlet or outlet horizontal adiabatic duct lengths is also investigated. The optimization is performed numerically by using the finite element technique, in the range 10⁵ < Ra < 10⁷ for Pr = 0.7, where Ra is the Rayleigh number based on a fixed total height H of the channel. The numerical results show that optimization is relevant, since the three degrees of freedom considered have a strong effect on the heat transfer delivered from the hot wall to the fluid. The optimal geometric characteristics obtained numerically (i.e., optimal spacing, optimal height and lengths) are reported and correlated within a 7.5% maximal disagreement range.     

An enhanced adsorption cycle operated by periodic reversal forced convection

A different cycle configuration is suggested in this work. Interesting improvement in performances is envisaged by applying the fixed bed adsorbers with the forced unsteady-state operation of periodic flow reversal to convective thermal wave cycle, which results in the so-called Periodic Reversal Forced Convective cycle. To describe process behavior for this kind of cycle with seven possible operating modes of flow reversal, a two-phase, one-dimensional model for the zeolite 13X-water pair is developed. Some important phenomena, such as the heat pipe effect, internal and external thermal regeneration etc., are discussed. The results of numerical simulations show that among all possible operating modes of flow reversal, only the mode of CCAA (switching the flow directions each phase duration) could improve the global performances of non-reversed flow operation, but others could not. Under the current conditions of cycle, the proposed CCAA cycle with heat exchanger regenerator could offer significant improvements in coefficient of performance (COP) over both the conventional cycle without flow reversal operation and existing convective thermal wave cycle with inert packed bed regenerator. The results of calculation also show that more than 1.7 of COP_h, 0.9 of COP_c and 125 W/kg adsorbent of specific cooling power (SCP) within this system could be possible. Analysis gives satisfactory explanation of these results.