Boiling heat transfer coefficients inside a vertical smooth tube for the water/lithium bromide mixture

This paper describes the experimental results obtained on the heat transfer in forced convective boiling for the water/lithium bromide mixture flowing upward in a vertical tube uniformly heated. The concentration range for the mixture was from 48.1 to 57.7 wt%. Correlations were proposed to correlate the experimental local heat transfer coefficients. The results showed that the local heat transfer coefficients are strongly dependent on Bo, 1/X_tt and 1/x at the analysed conditions. It was observed that the average heat transfer coefficients increased for the mixture with a decrease of the solution concentration or an increase of the mass flux. Copyright © 2002 John Wiley & Sons, Ltd.     

Heat transfer coefficients in two-phase flow for mixtures used in solar absorption refrigeration systems

This paper describes the experimental results obtained on the heat transfer in saturated nucleate boiling of refrigerant mixtures used in solar absorption refrigeration systems flowing upward in a uniformly heated vertical tube. The mixtures analysed were water/ammonia, ammonia/lithium nitrate and water/lithium bromide. The concentration range for the water/ammonia was from 38 to 48 wt%, for the ammonia/lithium nitrate was from 38 to 48 wt% and for the water/lithium bromide was from 48 to 58 wt%. It was observed that the average heat transfer coefficients increased for the mixtures with an increase in the quality, the heat flux and the solution concentration. Comparing the values of the average heat transfer coefficients for the three mixtures, it was observed that the highest values were obtained for the ammonia/water mixture.     

Single-stage and advanced absorption heat transformers operating with lithium bromide mixtures used to increase solar pond's temperature

Mathematical models of single-stage and advanced absorption heat transformers operating with the water/lithium bromide and water/Carrol™ mixtures were developed to simulate the performance of these systems coupled to a solar pond in order to increase the temperature of the useful heat produced by solar ponds. Plots of coefficients of performance and gross temperature lifts are shown against the temperatures of the heat supplied by the solar pond. The results showed that the single-stage and the double absorption heat transformer are the most promising configuration to be coupled to solar ponds. With single-stage heat transformers it is possible to increase solar pond's temperature until 50°C with coefficients of performance of about 0.48 and with double absorption heat transformers until 100°C with coefficients of performance of 0.33.     

Comparison of the theoretical performance of a solar air conditioning system operating with water/lithium bromide and an aqueous ternary hydroxide

This paper compares the theoretical performance of the modelling of a solar absorption air conditioning system operating with water/lithium bromide and an aqueous ternary hydroxide mixture consisting of sodium, potassium and cesium hydroxides in the proportions 40 : 36 : 24 (NaOH : KOH : CsOH). In this paper, plots of the coefficients of performance of a solar air conditioning system operating with these two mixtures are presented. The results showed that similar coefficients of performance are obtained for both mixtures, however, it was found that the system operating with the hydroxides may operate with a higher range of condenser and absorber temperatures and the heat delivered by these components can be removed by air.     

Optimization of energy plants including water/lithium bromide absorption chillers

In this paper a methodology for the optimal integration of water/lithium bromide absorption chillers in combined heat and power plants is proposed. This method is based on the economic optimization of an energy plant that interacts with a refrigeration cycle, by using a successive linear programming technique (SLP). The aim of this paper is to study the viability of the integration of already technologically available absorption chillers in CHP plants. The results of this alternative are compared with the results obtained using the conventional way of producing chilled water, that is, using mechanical vapour compression chillers in order to select the best refrigeration cycle alternative for a given refrigeration demand. This approach is implemented in the computer program XV, and tested using the data obtained in the water/LiBr absorption chiller of Bayer in Tarragona (Catalonia, Spain). The results clearly show that absorption chillers are not only a good option when low‐cost process heat is available, but also when a cogeneration system is present. In this latter case, the absorption chiller acts as a bottoming cycle by using steam generated in the heat recovery boiler. In this way, the cogeneration size can be increased producing higher benefits than those obtained with the use of compression chillers. Copyright © 2000 John Wiley & Sons, Ltd.     

Theoretical comparison of single stage and advanced absorption heat transformers operating with water/lithium bromide and water/Carrol mixtures

A thermodynamic analysis was carried out to compare the theoretical performance of single stage, two stage and double-absorption heat transformers operating with the water/lithium bromide and the water/Carrol mixtures, where Carrol is a mixture of lithium bromide and ethylene glycol [(CH₂OH)₂] in the ratio 1:4·5 by weight. A mathematical model to predict the theoretical performance of single stage and the advanced heat transformers is also described. Coefficients of performance and gross temperature lifts are compared for the different heat transformers and plotted against the main temperatures of the system for both mixtures. The water/Carrol mixture showed in general to have a better performance than the water/lithium bromide mixture. © 1998 John Wiley & Sons, Ltd.     

Thermodynamic analysis of a lithium bromide/water absorption system for cooling and heating applications

Absorption systems have the potential of employing thermal energy such as waste heat to produce both chilled water and hot water for building cooling and heating applications. In the present study, a lithium bromide/water (LiBr/H₂O) absorption system for cooling and heating applications was analysed on the basis of the first and second laws of thermodynamics. Simulation was employed to determine the coefficient of performance (COP) and the exergetic efficiency of the absorption system under different operating conditions such as the heat source, cooling water, chilled water, and supply hot water temperatures. Copyright © 2001 John Wiley & Sons, Ltd.     

Energy and exergy analysis of an experimental single‐stage heat transformer operating with the water/lithium bromide mixture

The first and second law of thermodynamics have been used to analyze the performance of an experimental single‐stage heat transformer operating with the water/lithium bromide mixture. Enthalpy coefficients of performance (COP), external coefficients of performance (COP_EXT), exergy coefficient of performance (ECOP), exergy destruction or irreversibility in the system and components (I) and the improvement potential (Pot) have been calculated against the gross temperature lift and the main operating temperatures of the system. The results showed that the highest COP, COP_EXT and ECOP values are obtained at the highest solution concentrations meanwhile the Pot and the I of the cycle remain almost constant against these parameters. Also it was shown that the COP, COP_EXT and ECOP decrease with an increase with the absorber temperature, meanwhile the Pot and the I increase. Moreover, it was observed that in all the cases independently of the operating temperatures of the system, the absorber accounts with most of the half of the total irreversibility in the system. Finally, it was shown that the improvement potential is considerable for the system. Copyright © 2009 John Wiley & Sons, Ltd.     

A droplet size dependent multiphase mixture model for two phase flow in PEMFCs

A droplet size dependent multiphase mixture model is developed in this paper, and the droplet size in the gas channel can be considered as a parameter in this multiphase mixture model, which includes the effect of gas diffusion layer (GDL) properties and the gas drag function and cannot be considered in the commonly used multiphase mixture model in the references. The three-dimensional two phase and non-isothermal simulation of the PEMFCs with a straight flow field is performed. The effect of droplet size on the liquid remove, the effect of liquid water on the heat transfer and the effect of gas flow pattern on the heat and mass transfer are mainly investigated. The simulation results show that the large droplet is hard to be dragged by the gas, so it produces large water saturation. The results of the heat transfer show that the liquid water hinders the heat transfer in the GDL and catalyst layer, so it produces the large relative high temperature area, and there are large temperature difference and water saturation in the PEMFCs operated with coflow pattern compared with counter flow pattern.     

Heat transfer characteristics of microencapsulated phase change material slurry in laminar flow under constant heat flux

Due to its large apparent specific heat during the phase change period, microencapsulated phase change material slurry (MPCMS) has been suggested as a medium for heat transfer. In this paper, the convective heat transfer characteristics of MPCMS flowing in a circular tube were experimentally and numerically investigated. The enhanced convective heat transfer mechanism of MPCMS, especially in the thermal fully developed range, was analyzed by using the enthalpy model. Three kinds of fluid–pure water, micro-particle slurry and MPCMS were numerically investigated. The results show that in the phase change heat transfer region the Ste number and the Mr number are the most important parameters influencing the Nusselt number fluctuation profile and the dimensionless wall temperature. Re_b, d_p and c also influence the Nusselt number profile and the dimensionless wall temperature, but they are independent of phase change process.     

Mass transfer behavior at horizontal cylinder in cross flow under different flow conditions

The rate of liquid-solid mass transfer at a horizontal cylinder placed in a cylindrical vertical column under different hydronamic conditions including gas sparging, single phase liquid flow, and two phase (gas-liquid) flow was studied experimentally by using the electrochemical technique which involves measuring the limiting current of the cathodic reduction of K₃Fe(CN)₆ using a solution containing 0.01 M K₃Fe(CN)₆ and 0.1 M K₄Fe(CN)₆ and a large excess of NaOH as supporting electrolyte. Variables studied were: liquid and gas superficial velocities, cylinder diameter, and solution physical properties. For gas sparging: the data were correlated for the conditions 0.2 < (Re.Fr) < 8.7, and 1253 < Sc < 2778 by the equation:

j=0.11(Re.Fr)−0.247.     

Heat transfer design in adsorption refrigeration systems for efficient use of low-grade thermal energy

Adsorption refrigeration and heat pump systems have been considered as important means for the efficient use of low-grade thermal energy of 60–150 °C. Sorption systems are merely thermodynamic systems based on heat exchangers, and therefore a good design to optimize heat and mass transfer with reaction or sorption processes is very important, for which the notable technique is the use of expanded graphite to improve both heat and mass transfer in the chemisorption beds. Studies have also shown the need to enhance the heat transfer in adsorption bed by matching with the efficient heat transfer of thermal fluids. Heat pipes and good thermal loop design coupled with adsorption beds could yield higher thermal performance of a sorption system. A novel design with passive evaporation, known as rising film evaporation coupled with a gravity heat pipe was introduced for high cooling output. It has also been shown that the performance of traditional heat and mass recovery in the sorption systems is limited, and novel arrangement of thermal fluid and refrigerant may improve the performance of sorption systems. Based upon the above researches, various sorption systems have been developed, and high performances have been reached.     

Heat transfer enhancement with the use of nanofluids in radial flow cooling systems considering temperature-dependent properties

Heat transfer enhancement capabilities of coolants with suspended metallic nanoparticles inside typical radial flow cooling systems are numerically investigated in this paper. The laminar forced convection flow of these nanofluids between two coaxial and parallel disks with central axial injection has been considered using temperature dependent nanofluid properties. Results clearly indicate that considerable heat transfer benefits are possible with the use of these fluid/solid particle mixtures. For example, a Water/Al₂O₃ nanofluid with a volume fraction of nanoparticles as low as 4% can produce a 25% increase in the average wall heat transfer coefficient when compared to the base fluid alone (i.e., water). Furthermore, results show that considerable differences are found when using constant property nanofluids (temperature independent) versus nanofluids with temperature dependent properties. The use of temperature-dependent properties make for greater heat transfer predictions with corresponding decreases in wall shear stresses when compared to predictions using constant properties. With an increase in wall heat flux, it was found that the average heat transfer coefficient increases whilst the wall shear stress decreases for cases using temperature-dependent nanofluid properties.     

Heat transfer and flow characteristics in the hard disk drive tester

The numerical results of the heat transfer and flow characteristics in the hard disk drive tester are presented. The testing of the hard disk drive with keeping drives within the normal and high temperatures in the tester has been introduced as one of the manufacturing processes of the hard disk drive. The cooling air entering the tester is induced by the 10 axial fans into the tester and is impinged the hard disk drives and then discharged to the atmosphere. The k–ε standard turbulent model is applied to analyze the model. The results obtained from the model are verified by comparing with the measured data. Reasonable agreement is obtained from the comparison between the results obtained from the model and those from the experiment. The numerical results show that the flow and temperature distribution of cooling air are not uniformed. Which none-uniform temperature and accumulated heat are significantly factors to the failure of the hard disk drives. The results of this study are of technology importance for the efficient design and/or approved hard disk drive tester to decrease hard disk drive failure.     

Comparison of instrumental methods for in-line determination of LiBr concentration in solar absorption thermal systems

This paper shows the comparison of the models for the in-line determination of the concentration of aqueous lithium bromide in the components of photo-thermal systems. For this study several operational conditions at different concentrations, pressures and temperatures have been considered. The behaviors of the physical properties have been analyzed as a function of the temperature. The results show the advantage of the use of the refractive index compared to the data of the thermal conductivity. The in-line determination of LiBr concentration allows calculations to improve optimal recovery of solar heat in heat transformers or air conditioning systems.     

Heat transfer and turbulent flow friction in a circular tube fitted with conical-nozzle turbulators

The paper deals with an experimental study of the influence of conical-nozzle turbulator inserts on heat transfer and friction characteristics in a circular tube. In the present work, the turbulators are placed in the test tube section with two different types: (1) diverging nozzle arrangement (D-nozzle turbulator) and (2) converging nozzle arrangement (C-nozzle turbulator). The turbulators are thoroughly inserted inside the tube with various pitch ratios, PR = 2.0, 4.0, and 7.0. The Reynolds number based on the bulk average properties of the air is in a range of 8000 to 18,000 and the experimental data obtained are compared with those obtained from the plain tube and from the literature. The experimental results reveal that increasing the Reynolds number at a given pitch ratio of the turbulators leads to the significant increase in Nusselt number indicating enhanced heat transfer coefficient due to rising convection as the flow increases. However, the friction factor at a given Reynolds number considerably increases with the reduction of pitch ratio and Reynolds number. The D-nozzle arrangement, creating stronger reverse/turbulence flow, provides higher the heat transfer rate and friction factor than the C-nozzle arrangement. The heat transfer rates obtained from using both nozzle-turbulators, in general, are found to be higher than that from the plain tube at a range of 236 to 344%, depending on Reynolds number and the turbulator arrangements. In addition, proposed correlations from the present experimental data for Nusselt number and friction factor are also presented.     

An experimental study in determining the local heat transfer coefficients for the plate finned-tube heat exchangers

A three-dimensional inverse problem in determining the local heat transfer coefficients for the plate finned-tube heat exchangers utilizing the steepest descent method (SDM) and a general purpose commercial code CFX4.4 is applied successfully in the present study based on the measured temperature distributions on fin surface by infrared thermography.Two different tube arrangements (i.e. in-line and staggered) with different fin pitch and air velocity are considered and the corresponding local heat transfer coefficients are to be determined. Results show that some interesting phenomena of the local heat transfer coefficients for the finned surface are found in the work and the averaged heat transfer coefficient of the staggered configuration is about 8–13% higher than that of the in-line configuration.     

Prediction of friction factors and heat transfer coefficients for turbulent flow in corrugated tubes combined with twisted tape inserts. Part 2: heat transfer coefficients

A simple mathematical model following the suggestion of Smithberg and Landis has been created to predict the heat transfer coefficients for the case of a fully developed turbulent flow in a spirally corrugated tube combined with a twisted tape insert. The heat transfer can be predicted from the combined effects of the axial and the tangential boundary layer flows coupled with an additional “vortex mixing” effect near the wall through the solution of the corresponding momentun and energy transfer equations. The “wall roughness” has an effect simultaneously on the axial velocity, secondary fluid motion and the resulting swirl mixing. The model reflects the influence of the “wall roughness” and the twisted tape on the thermal resistances of the helicoidal core flow, twisting boundary layer flow and the viscous sublayer near the wall. The calculated heat transfer coefficients have been compared to 544 experimental points obtained from 57 tubes tested. Four hundred thirty-eight points (80.5%) have a relative difference of less than ±15% and 106 points (19.5%) have a relative difference between ±(15-20)%.     

Heat and mass transfer analysis for boundary layer stagnation-point flow towards a heated porous stretching sheet with heat absorption/generation and suction/blowing

A similarity analysis is performed to investigate the structure of the boundary layer stagnation-point flow and heat transfer over a stretching sheet in a porous medium subject to suction/blowing and in the presence of internal heat generation/absorption. A scaling group of transformations is applied to get the invariants. Using the invariants, a third and a second order ordinary differential equations corresponding to the momentum and energy equations are obtained respectively. Boundary layer velocity and temperature profiles are determined numerically for various values of the ratio of free stream velocity and stretching velocity, the permeability parameter, suction/blowing parameter, heat source/sink parameter, Prandtl number. It is found that the horizontal velocity increases with the increasing value of the ratio of the free stream velocity (ax) and the stretching velocity (cx). The temperature decreases in this case. At a particular point of the porous stretching sheet, the non-dimensional fluid velocity decreases with the increase of the permeability of the porous medium and also with the increasing suction parameter when the free stream velocity is less than stretching velocity whereas fluid velocity increases with the increasing injection parameter. But when the free stream velocity is greater than the stretching velocity the opposite behaviour of horizontal velocity is noticed. The dimensionless temperature at a point of the sheet decreases due to suction but increases due to injection. The temperature at a point is found to decrease with the increasing Prandtl number.