Thermophysical properties of the trifluoroethanol-pyrrolidone system for absorption heat transformers

Thermophysical properties of the 2,2,2-trifluoroethanol-2-pyrrolidone system have been investigated. The scope of the study includes density, viscosity, thermal conductivity, the equilibrium vapour pressure-concentration-temperature relationship, specific heat capacity, heat of mixing and enthalpy. Experimental measurements were carried out. Mathematical correlations for various properties were mainly derived from measured data. The good combination of physical and thermal properties of trifluoroethanol and pyrrolidone shows that they can be used as a working pair for absorption heat transformers.     

Predicting heat pump performance by thermodynamic generalizations of fluid flow and heat transfer data

Thermodynamic generalizations based on reduced pressure proposed in the 1960s are reviewed and updated to reflect the current state of the art. The application of the method is illustrated by analytical and numerical examples and an assessment made of its value in heat exchanger design practice with special emphasis on two-phase forced convection refrigeration cycle applications. It is shown that this thermodynamic approach provides the heat exchanger designer, and to some extent the system engineer with an additional tool which is simple, effective and above all more reliable, particularly in evaporator and condenser design practice, than current conventional semi-empirical correlations.     

Operational performance of a cryogenic loop heat pipe with insufficient working fluid inventory

A cryogenic loop heat pipe (CLHP) has been developed for future aerospace applications at the Technical Institute of Physics and Chemistry (TIPC). It has been demonstrated that this CLHP, when placed horizontally, can operate in liquid-nitrogen temperature range and have a heat transfer capability of up to 12 W with proper working fluid inventory. This paper presents some particular characteristics of the CLHP when the compensation chamber is half-filled with liquid-phase working fluid before startup. The device has been tested at different orientations using nitrogen as the working fluid in order to compare its thermal behavior, specially related to the heat transfer capability, the operation temperature and the thermal resistance, as well as to investigate its operational characteristics under power level as low as 1 W. Tests were performed for the CLHP at horizontal position and with the liquid line 3.4 and 6.4 cm below the vapor line, respectively. The experimental results show the operationability of the CLHP tested at three orientations and tests with the liquid line 6.4 cm below the vapor line show lower operation temperatures and higher heat transfer capability.     

Heat capacity of the water-lithium bromide system and the water-lithium bromide-zinc bromide-lithium chloride system at high temperatures

The heat capacities of the water-lithium bromide system were measured in the temperature range 313.15–433.15 K. Those of the water-lithium bromide-zinc bromide-lithium chloride system wer also measured in the temperature range 373.15–433.15 K. On the basis of the experimental data, an empirical formula for the water-lithium bromide system was obtained by the least-squares method. The calculated values from the empirical formula were in good agreement with literature values obtained in the range of low temperatures. In addition, the heat capacity data obtained for the water-lithium bromide-zinc bromide-lithium chloride system were compared with the calculated values from Ueda's equation which is applicable at low temperatures, and they agreed well.     

Carbon dioxide as a working fluid in drying heat pumpsLe dioxyde de carbone comme fluide actif dans des séchoirs à pompe à chaleur

Carbon dioxide as a working fluid in refrigeration and heat pump systems is increasingly important in view of the CFC substitution problem. It is both under ecological and economical aspects an attractive alternative to the HFC working fluids at present in practical use. The thermophysical properties and characteristics of carbon dioxide are quite different from those of refrigerants used in conventional vapour compression cycles. Its application in conventional vapour compression refrigerating systems is limited by its critical parameters (t_c=31.1°C and p_c=73.8 bar). The possibility to use carbon dioxide also beyond these limits in high temperature processes, e.g. heat pumps, is given by the application of a trans-critical process. The design and construction of a commercial drying heat pump system (batch type cabinet dryer with 12 kW heating capacity and closed air circuit) using the natural working fluid carbon dioxide is shown and experimental results of investigations carried out are presented. Energy savings are given compared to manufacturer's data of energy consumption.     

Dynamics of a heat transformer working with the mixture NaOH---H2O

The model described in the paper is based on the balance equations for mass and energy for the different apparatus such as absorber, generator, evaporator, condenser and solution heat exchanger. It simulates a heat transformer process and ensures calculations not too time-consuming and much faster than in real time. With the aid of the model the transient behaviour of a heat transformer working with the mixture NaOH---H₂O is examined. For steady state operation the simulations agree well with experiments on a pilot plant. With NaOH---H₂O as working substances high COPS up to 0.49 were obtained with this heat transformer. For unsteady state operation the deviations between experiments and simulations were slightly higher than those for steady state. The model nevertheless predicts the transient behaviour with a satisfying accuracy. Also the startup and shutdown behaviour of a heat transformer was investigated under different conditions.     

Thermodynamic properties for the quaternary system H2O + CH3OH + LiBr + ZnCl2

The thermodynamic properties (solubility, vapour pressure, density, viscosity, heat capacity and heat of mixing) of the H₂O + CH₃OH + LiBr + ZnCl₂ (9:1 H₂O:CH₃OH and 1:1 LiBr:ZnCl₂ by mass) system using H₂O + CH₃OH as the working media and LiBr + ZnCl₂ as the absorbents were measured. The solubility data were obtained in the temperature range from 270.35 to 389.55 K. The measurements of vapour pressure, density, viscosity and heat capacity were carried out at various temperatures and absorbent concentrations. The differential heat of dilution and differential heat of solution at 298.15 K were measured for solutionw with absorbent concentrations from 0 to 75.2 wt%. The integral heat of mixing data at 298.15 K were obtained from both sets of experimental data. The integral heats of mixing for this quaternary system showed exothermic behaviour. The vapour pressure data were correlated with an Antoine-type equation. An empirical formula for the heat capacity was obtained from experimental data. The experimental data for the basic thermodynamic properties of this quaternary system were compared with those of the basic H₂O + LiBr system.     

Effect of strip location on the air-side pressure drop and heat transfer in strip fin-and-tube heat exchangerEffet de la localisation des gaufrures d’ailettes sur la chute de pression de l''air et sur le transfert de chaleur dans un échangeur à tubes à ailettes

This work discusses about the pressure drop and heat transfer characteristics of a plain fin, a strip fin and combinations thereof in the fin-and-tube heat exchanger. Experimental apparatus and test algorithm using scale-up model fin geometry are discussed. The present work uses the electric heaters inside of the tube as the heating source to simulate the hot water loop in the wind tunnel test. The test results of the plain and strip fins are compared with the existing correlation and experimental data. It was found that a hybrid fin, plain fin at front row and strip fin at rear row, was more effective to enhance the heat transfer than that of the whole strip fin at the same fan power.

Résumé

Dans l’article, on traite des caractéristiques de la chute de pression et du transfert de chaleur d’une ailette lisse, d’une ailette gaufrée et de combinaisons de ces deux types d’ailettes dans un échangeur de chaleur à tubes ailetés. Le dispositif expérimental et l’algorithme utilisant une géométrie des ailettes échelonnées sont présentés. On utilise des équipements de chauffage électriques à l’intérieur du tube, comme source de chaleur, pour simuler la boucle d’eau chaude dans le tunnel expérimental. Les résultats expérimentaux concernant les ailettes lisses et les ailettes gaufrées sont comparés avec la corrélation existante et les données expérimentales. On a observé que des ailettes hybrides, c’est-à-dire lisses à la rangée frontale et gaufrées à la rangée arrière, étaient plus efficaces pour intensifier le transfert de chaleur que des ailettes entièrement gaufrées, à la même puissance du ventilateur.     

Numerical simulation of an advanced energy storage system using H2O–LiBr as working fluid, Part 1: System design and modeling

The advanced energy storage technology proposed and patented by authors can be applied for cooling, heating, dehumidifying, combined cooling and heating, and so on. It is also called the variable mass energy transformation and storage (VMETS) technology in which the masses in one or two storage tanks change continuously during the energy charging and discharging processes. This paper presents an advanced energy storage system using aqueous lithium bromide (H₂O–LiBr) as working fluid. As one of VMETS systems, this system is a closed system using two storage tanks. It is used to shift electrical load and store energy for cooling, heating or combined cooling and heating. It is environmental friendly because the water is used as refrigerant in the system. Its working principle and process of energy transformation and storage are totally different from those of the traditional thermal energy storage (TES) systems. The electric energy in off-peak time is mostly transformed into the chemical potential of the working fluid and stored in the system firstly. And then the potential is transformed into cold or heat energy by absorption refrigeration or heat pump mode when the consumers need the cold or heat energy. The key to the system is to regulate the chemical potential by controlling the absorbent (LiBr) mass fraction or concentration in the working fluid with respect to time. As a result, by using a solution storage tank and a water storage tank, the energy transformation and storage can be carried out at the desirable time to shift electric load efficiently. Since the concentration of the working solution in the VMETS cycle varies continuously, the working process of the VMETS system is dynamic. As the first part of our study, the working principle and flow of the VMETS system were introduced first, and then the system dynamic models were developed. To investigate the system characteristics and performances under full-storage and partial-storage strategies, the numerical simulation will be performed in the subsequent paper. The simulation results will be very helpful for guiding the actual system and device design.     

Analysis of a hybrid compression—absorption cycle using lithium bromide and water as the working fluid

Hybrid refrigeration cycles which combine a mechanical compressor and an absorption cycle in such a way that they share a single evaporator were analysed. The motivation for the investigation of hybrid cycles was the need to more efficiently utilize the output of an internal combustion engine. The hybrid cycles make efficient use of both the work and the heat output of an engine. Performance calculations are reported for a promising cycle which utilizes LiBr-H₂O as the working fluid. For this working fluid, the refrigerant is water. Owing to the potential sensitivity of the absorption cycle components to oil contamination, the cycle was analysed assuming an oil-free steam compressor (screw design). Although oil-free steam compressors are available, they are used only sparingly in the industry. The capital cost for such a compressor is very high and the isentropic efficiency of the available units is low. This combination of high cost and low performance results in poor economics for the hybrid cycle based on the available technology. However, the cycle has significant potential from a thermodynamic viewpoint and it provides an incentive for compressor manufacturers to refine the oil-free steam compression technology.     

Numerical simulation of an advanced energy storage system using H2O–LiBr as working fluid, Part 2: System simulation and analysis

This paper is the second part of our study on the advanced energy storage system using H₂O–LiBr as working fluid. In the first part, the system working principle has been introduced, and the system dynamic models in the operation process have also been developed. Based on the previous research, this paper focuses on the numerical simulation to investigate the system dynamic characteristics and performances when it works to provide combined air-conditioning and hot water supplying for a hotel located near by Yangzi River in China. The system operation conditions were set as follows: the outdoor temperature was between 29 °C and 38 °C, the maximum air-conditioning load was 1450 kW, the total air-conditioning capacity was 19,890 kWh and the 50 °C hot water capacity for showering was 20 tons which needed heat about 721 kWh on a given day. Under these conditions, the system operation characteristics were simulated under the full- and partial-storage strategies. The simulation results predicted the dynamic characteristics and performances of the system, including the temperature and concentration of the working fluid, the mass and energy in the storage tanks, the compressor intake mass or volume flow rate, discharge pressure, compression ratio, power and consumption work, the heat loads of heat exchanger devices in the system and so on. The results also showed that the integrated coefficient of performances (COP_int) of the system were 3.09 and 3.26, respectively, under the two storage strategies while the isentropic efficiency of water vapor compressor was 0.6. The simulation results are very helpful for understanding and evaluating the system as well as for system design, operation and control, and device design or selection in detail.     

Heat transfer characteristics of flat plate finned-tube heat exchangers with large fin pitch

The objective of this study is to provide experimental data that can be used in the optimal design of flat plate finned-tube heat exchangers with large fin pitch. In this study, 22 heat exchangers were tested with a variation of fin pitch, number of tube row, and tube alignment. The air-side heat transfer coefficient decreased with a reduction of the fin pitch and an increase of the number of tube row. The reduction in the heat transfer coefficient of the four-row heat exchanger coil was approximately 10% as the fin pitch decreased from 15.0 to 7.5 mm over the Reynolds number range of 500–900 that was calculated based on the tube diameter. For all fin pitches, the heat transfer coefficient decreased as the number of tube row increased from 1 to 4. The staggered tube alignment improved heat transfer performance more than 10% compared to the inline tube alignment. A heat transfer correlation was developed from the measured data for flat plate finned-tubes with large fin pitch. The correlation yielded good predictions of the measured data with mean deviations of 3.8 and 6.2% for the inline and staggered tube alignment, respectively.     

Heat and mass transfer enhancement of binary nanofluids for H2O/LiBr falling film absorption process

The objectives of this study are to measure the vapor absorption rate and heat transfer rate for falling film flow of binary nanofluids, and to compare the enhancement of heat transfer and mass transfer under the same conditions of nanofluids. The key parameters are the base fluid concentration of LiBr, the concentration of nanoparticles in weight %, and nanoparticle constituents. The binary nanofluids are H₂O/LiBr solution with nanoparticles of Fe and Carbon nanotubes (CNT) with the concentrations of 0.0, 0.01 and 0.1 wt %. The vapor absorption rate increases with increasing the solution mass flow rate and the concentration of Fe and CNT nanoparticles. It is found that the mass transfer enhancement is much more significant than the heat transfer enhancement in the binary nanofluids with Fe and CNT. It is also found that the mass transfer enhancement from the CNT nanoparticles becomes higher than that from the Fe nanoparticles. Therefore, the CNT is a better candidate than Fe nanoparticles for absorption performance enhancement in H₂O/LiBr absorption system.     

Heat transfer enhancement by Marangoni convection in the NH3–H2O absorption process

The objectives of this paper are to quantify the effect of Marangini convection on the absorption performance for the ammonia–water absorption process, and to visualize Marangoni convection that is induced by adding a heat transfer additive, n-octanol. A real-time single-wavelength holographic interferometer is used for the visualization using a He–Ne gas laser. The interface temperature is always the highest due to the absorption heat release near the interface. It was found that the thermal boundary layer (TBL) increased faster than the diffusion boundary layer (DBL), and the DBL thickness increased by adding the heat transfer additive. At 5 s after absorption started, the DBL thickness for 5 mass% NH₃ without and with the heat transfer additive was 3.0 and 4.5 mm, respectively. Marangoni convection was observed near the interface only in the cases with heat transfer additive. The Marangoni convection was very strong just after the absorption started and it weakened as time elapsed. It was concluded that the absorption performance could be improved by increasing the absorption driving potential (x_vb−x_vi) and by increasing the heat transfer additive concentration. The absorption heat transfer was enhanced as high as 3.0–4.6 times by adding the heat transfer additive that generated Marangoni convection.     

Experimental correlation of combined heat and mass transfer for NH3–H2O falling film absorptionCorrélation expérimentale entre le transfert de chaleur et de masse pour de l'absorption au NH3–H2O à film tombant

In this article, experimental analysis was performed for ammonia–water falling film absorption process in a plate heat exchanger with enhanced surfaces such as offset strip fin. This article examined the effects of liquid and vapor flow characteristics, inlet subcooling of the liquid flow and inlet concentration difference on heat and mass transfer performance. The inlet liquid concentration was selected as 5%, 10% and 15% of ammonia by mass while the inlet vapor concentration was varied from 64.7% to 79.7%. It was found that before absorption started, there was a rectification process at the top of the test section by the inlet subcooling effect. Water desorption phenomenon was found near the bottom of the test section. It was found that the lower inlet liquid temperature and the higher inlet vapor temperature, the higher Nusselt and Sherwood numbers are obtained. Nusselt and Sherwood number correlations were developed as functions of falling film Reynolds Re₁, vapor Reynolds number Re_v, inlet subcooling and inlet concentration difference with ±15% and ±20% error bands, respectively.     

Heat pumps in the USA

First commercially introduced in the 1930s, heat pumps are now used to heat, or heat and cool, approximately 30% of all new buildings, both residential and commercial, in the USA. Their use is considerably lower for domestic water and industrial process heating, and is still at the pioneering stage for district heating and cooling. Widespread installation of air conditioning, prevalent use of air rather than water for heat distribution, and generally lower energy prices result in major differences in heat pump acceptance compared to that in European countries. This Paper surveys the current status of heat pumps in the USA and factors influencing their future.     

Flow boiling heat transfer characteristics of CO2 at low temperatures

This paper presents an overview of the flow boiling heat transfer characteristics and the special thermo-physical properties of CO₂ at low temperatures (down to −30 °C). Subsequently, the boiling heat transfer of CO₂ at low temperatures is experimentally investigated in a horizontal tube with inner diameter of 4.57 mm. Due to the large surface tension, the boiling heat transfer coefficient of CO₂ is found to be much lower at low temperatures but it increases with vapour quality (until dryout), which is contrary to the trend at high temperatures around 0 °C. None of the empirical correlations from open literature were able to predict the boiling heat transfer coefficient for CO₂ in good agreement with the experimental data, suggesting the need for further research in this area.     

Compression heat pump with solution circuit Part 2: sensitivity analysis of construction and control parameters

In this Paper the results of a sensitivity analysis of a number of construction and control parameters of a compression heat pump with solution circuit (CHSC) are presented. The analysis is calculated with a computer model which is verified by the experimental results from a test plant.     

CO2 flow condensation heat transfer and pressure drop in multi-port microchannels at low temperatures

CO₂ flow condensation heat transfer coefficients and pressure drop are investigated for 0.89 mm microchannels at horizontal flow conditions. They were measured at saturation temperatures of −15 and −25 °C, mass fluxes from 200 to 800 kg m⁻² s⁻¹, and wall subcooling temperatures from 2 to 4 °C. Flow patterns for experimental conditions were predicted by two flow pattern maps, and it could be predicted that annular flow patterns could exist in most of flow conditions except low mass flux and low vapor quality conditions. Measured heat transfer coefficients increased with the increase of mass fluxes and vapor qualities, whereas they were almost independent of wall subcooling temperature changes. Several correlations could predict heat transfer coefficients within acceptable error range, and from this comparison, it could be inferred that the flow condensation mechanism in 0.89 mm channels should be similar to that in large tubes. CO₂ two-phase pressure drop, measured in adiabatic conditions, increased with the increase of mass flux and vapor quality, and it decreased with the increase of saturation temperature. By comparing measured pressure drop with calculated values, it was shown that several correlations could predict the measured values relatively well.     

Modelling of the heat exchangers of a small capacity, hot water driven, air-cooled H2O–LiBr absorption cooling machine

General models for the design of the heat exchangers (absorber, generator, condenser and evaporator) of a prototype of an air-cooled absorption chiller of 2 kW for air-conditioning using the pair H₂O–LiBr have been developed. An absorption machine of such characteristics has been constructed to be used as a test facility for validating the results obtained from the mathematical models developed. The discrepancies considering the heat exchanged between numerical results and experimental data are under 15% in most cases for all these components except the condenser, where the discrepancies are higher. The conclusions reported will lead to: (i) future improvements of the mathematical simulation models and (ii) improvements in the experimental infrastructure.