A minichannel aluminium tube heat exchanger – Part II: Evaporator performance with propane

This paper presents heat transfer data for a multiport minichannel heat exchanger vertically mounted as an evaporator in a test-rig simulating a small water-to-water heat pump. The multiport minichannel heat exchanger was designed similar to a shell-and-tube type heat exchanger, with a six-channel tube of 1.42 mm hydraulic diameter, a tube-side heat transfer area of 0.777 m² and a shell-side heat transfer area of 0.815 m². Refrigerant propane with a desired vapour quality flowed upward through the tubes and exited with a desired superheat of 1–4 K. A temperature-controlled glycol solution that flowed downward on the shell-side supplied the heat for the evaporation of the propane. The heat transfer rate between the glycol solution and propane was controlled by varying the evaporation temperature and propane mass flow rate while the glycol flow rate was fixed (18.50 l min⁻¹). Tests were conducted for a range of evaporation temperatures from −15 to +10 °C, heat flux from 2000 to 9000 W m⁻² and mass flux from 13 to 66 kg m⁻² s⁻¹. The heat transfer coefficients were compared with 14 correlations found in the literature. The experimental heat transfer coefficients were higher than those predicted by many of the correlations. A correlation which was previously developed for a very large and long tube (21 mm diameter and 10 m long) was in good agreement with the experimental data (97% of the data within ±30%). Several other correlations were able to predict the data within a reasonable deviation (within ±30%) after some adjustments to the correlations.     

Composite adsorbent of CaCl2 and expanded graphite for adsorption ice maker on fishing boats

Adsorption performances and thermal conductivity were tested for three types of adsorbent: Pure CaCl₂ powder, simple composite adsorbent and consolidated composite adsorbent. The simple composite adsorbents show better adsorption performance because the additive of expanded graphite in CaCl₂ powder has restrained the agglomeration phenomenon in adsorption process and improved the adsorption performance of CaCl₂. The consolidated composite adsorbent are suitable to be used as adsorbent for ice maker on fishing boats because they have higher thermal conductivity, larger volumetric cooling capacity, higher SCP values and better anti-sway performance than simple composite adsorbents. Thermal conductivity of the consolidated composite adsorbent is 6.5–9.8 W m⁻¹ K⁻¹ depending on the molding pressure, ranging from 5 to 15 MPa, which is about 32 times higher than the thermal conductivity of CaCl₂ powder. The volumetric cooling capacity of consolidated composite adsorbent is about 52% higher than the best result obtained for CaCl₂ at the evaporating temperature of −10 °C. The SCP of the consolidated adsorbent increases of about 353% than CaCl₂ powder from simulation results at T_ad=30 °C and T_ev=−10 °C. The consolidated composite adsorbents have good anti-sway performance and they are not easy to be scattered out when the fishing boats sway on the sea.     

Convective boiling performance of refrigerant R-134a in herringbone and microfin copper tubes

This paper reports an experimental investigation of convective boiling heat transfer and pressure drop of refrigerant R-134a in smooth, standard microfin and herringbone copper tubes of 9.52 mm external diameter. Tests have been conducted under the following conditions: inlet saturation temperature of 5 °C, qualities from 5 to 90%, mass velocity from 100 to 500 kg s⁻¹ m⁻², and a heat flux of 5 kW m⁻². Experimental results indicate that the herringbone tube has a distinct heat transfer performance over the mass velocity range considered in the present study. Thermal performance of the herringbone tube has been found better than that of the standard microfin in the high range of mass velocities, and worst for the smallest mass velocity (G=100 kg s⁻¹ m⁻²) at qualities higher than 50%. The herringbone tube pressure drop is higher than that of the standard microfin tube over the whole range of mass velocities and qualities. The enhancement parameter is higher than one for both tubes for mass velocities lower than 200 kg s⁻¹ m⁻². Values lower than one have been obtained for both tubes in the mass velocity upper range as a result of a significant pressure drop increment not followed by a correspondent increment in the heat transfer coefficient.     

Influence of mist-chilling on post-harvest quality of fresh strawberries Cv. Mara des Bois and Gariguette

The aim of this study was to assess the impact of mist-chilling on high-grade strawberry post-harvest quality (Cultivars “Gariguette” and “Mara des Bois”). Strawberries were chilled at 2 °C using three processes: air blast chilling at 0.3 m s⁻¹ or 1 m s⁻¹ and mist-chilling at 1 m s⁻¹. After chilling, fruits were submitted to different distribution chains characterised by different handling conditions and storage temperatures (2 °C or 7 °C) and by a 12 h retailing step at 20 °C. Strawberry quality was assessed by measuring 7 parameters: weight loss, commercial loss, firmness, sugar content, acidity, colour and sensory quality. Compared to air-chilling, mist-chilling did not reduce chilling time but it reduced weight loss by 20–40%. Mist-chilling had no detrimental effect on commercial loss defined as the percentage of fruit more than 1/3 of surface affected. It did not induce any major changes on strawberry quality. Temperature fluctuations undergone during cold storage and retailing had a detrimental effect on weight loss. The beneficial effect of packaging on weight loss was confirmed.     

An experimental solar-powered adsorptive refrigerator tested in Burkina-Faso

An adsorptive solar refrigerator was built and tested in May 1999 in Ouagadougou, Burkina-Faso. The adsorption pair is activated carbon + methanol. The adsorber is also the solar collector (2 m², single glazed), the condenser is air-cooled (natural convection) and the evaporator contains 40 l of water that can freeze into ice. This amount of ice acts as a cold storage for the cold cabinet (available volume of 440 l). Elements such as valves and a graduated bottle are installed, but only for experimental purposes. Apart from these valves, and also ventilation dampers which are open at night time and closed at daytime, the machine does not contain any moving parts and does not consume any mechanical energy. Within the requirement of vacuum technology, the machine is relatively easy to manufacture, so that construction in Burkina-Faso is feasible. Experimental performance is presented in terms of gross solar COP. During the test period, irradiance were quite good (between 19 and 25 MJ m⁻²), but the ambient temperature was relatively warm (averagely 27.4 °C at sunrise and 37.4 °C at mid-afternoon). The experimental values of the gross solar COP lie between 0.09 and 0.13. Despite a warm climate, the performance of the machine compares favourably to previously published results.

Résumé

An adsorptive solar refrigerator was built and tested in May 1999 in Ouagadougou, Burkina-Faso. The adsorption pair is activated carbon + methanol. The adsorber is also the solar collector (2 m², single glazed), the condenser is air-cooled (natural convection) and the evaporator contains 40 l of water that can freeze into ice. This amount of ice acts as a cold storage for the cold cabinet (available volume of 440 l). Elements such as valves and a graduated bottle are installed, but only for experimental purposes. Apart from these valves, and also ventilation dampers which are open at night time and closed at daytime, the machine does not contain any moving parts and does not consume any mechanical energy. Within the requirement of vacuum technology, the machine is relatively easy to manufacture, so that construction in Burkina-Faso is feasible. Experimental performance is presented in terms of gross solar COP. During the test period, irradiance were quite good (between 19 and 25 MJ m⁻²), but the ambient temperature was relatively warm (averagely 27.4 °C at sunrise and 37.4 °C at mid-afternoon). The experimental values of the gross solar COP lie between 0.09 and 0.13. Despite a warm climate, the performance of the machine compares favourably to previously published results.     

Freezing time and rate of cauliflower florets under different freezing conditions

In this study, the freezing time and rate of 1 cm³ cauliflower floret samples were determined under different freezing conditions in an air blast freezer. Four different air temperatures (−20, −25, −30 and −35°C) and six different air velocities (70, 131, 189, 244, 280 and 293 m min⁻¹) were applied in the freezer, and the freezing rate and time of cauliflower pieces were determined under each condition. The freezing time of cauliflower samples frozen with cold air at −20°C and 280 m min⁻¹ was similar to that of samples frozen with cold air at −35°C and 70 m min⁻¹. When the velocity of air was increased from 70 m min⁻¹ to 293 m min⁻¹, the freezing time was approximately halved.     

NH3 in-tube condensation heat transfer and pressure drop in a smooth tube

This paper presents an overview of the issues and new results for in-tube condensation of ammonia in horizontal round tubes. A new empirical correlation is presented based on measured NH₃ in-tube condensation heat transfer and pressure drop by Komandiwirya et al. [Komandiwirya, H.B., Hrnjak, P.S., Newell, T.A., 2005. An experimental investigation of pressure drop and heat transfer in an in-tube condensation system of ammonia with and without miscible oil in smooth and enhanced tubes. ACRC CR-54, University of Illinois at Urbana-Champaign] in an 8.1 mm aluminum tube at a saturation temperature of 35 °C, and for a mass flux range of 20–270 kg m⁻² s⁻¹. Most correlations overpredict these measured NH₃ heat transfer coefficients, up to 300%. The reasons are attributed to difference in thermophysical properties of ammonia compared to other refrigerants used in generation and validation of the correlations. Based on the conventional correlations, thermophysical properties of ammonia, and measured heat transfer coefficients, a new correlation was developed which can predict most of the measured values within ±20%. Measured NH₃ pressure drop is shown and discussed. Two separated flow models are shown to predict the pressure drop relatively well at pressure drop higher than 1 kPa m⁻¹, while a homogeneous model yields acceptable values at pressure drop less than 1 kPa m⁻¹. The pressure drop mechanism and prediction accuracy are explained though the use of flow patterns.     

Study on an activated carbon fiber–ethanol adsorption chiller: Part I – system description and modelling

This article presents the transient modelling for a two-bed, activated carbon fiber (ACF)–ethanol adsorption chiller. This innovative adsorption chiller employs pitch based ACF of type A-20 as adsorbent which is a fibrous adsorbent having the advantages of fast adsorption rate, high porosity and ease of handling when compared with granular adsorbents and powdered adsorbents. Ethanol is used as refrigerant as it has no harm to environment, it is a non-toxic substance, moreover, ethanol has comparatively higher vapor pressure even at low temperature. This innovative system utilizes effectively low-temperature waste heat sources of temperature between 60 and 95 °C along with a coolant at 30 °C. We have found that, regardless of the initial mass distribution, the ACF–ethanol adsorption chiller is able to achieve the same cyclic-steady-state within three cycles or 1890 s.     

A minichannel aluminium tube heat exchanger – Part III: Condenser performance with propane

This paper reports heat transfer results obtained during condensation of refrigerant propane inside a minichannel aluminium heat exchanger vertically mounted in an experimental setup simulating a water-to-water heat pump. The condenser was constructed of multiport minichannel aluminium tubes assembled as a shell-and-tube heat exchanger. Propane vapour entered the condenser tubes via the top end and exited sub-cooled from the bottom. Coolant water flowed upward on the shell-side. The heat transfer areas of the tube-side and the shell-side of the condenser were 0.941 m² and 0.985 m², respectively. The heat transfer rate between the two fluids was controlled by varying the evaporation temperature while the condensation temperature was fixed. The applied heat transfer rate was within 3900–9500 W for all tests. Experiments were performed at constant condensing temperatures of 30 °C, 40 °C and 50 °C, respectively. The cooling water flow rate was maintained at 11.90 l min⁻¹ for all tests. De-superheating length, two-phase length, sub-cooling length, local heat transfer coefficients and average heat transfer coefficients of the condenser were calculated. The experimental heat transfer coefficients were compared with predictions from correlations found in the literature. The experimental heat transfer coefficients in the different regions were higher than those predicted by the available correlations.     

Modelling a ground-coupled heat pump system using adaptive neuro-fuzzy inference systems

The aim of this study is to demonstrate the usefulness of an adaptive neuro-fuzzy inference system (ANFIS) for the modelling of ground-coupled heat pump (GCHP) system. The GCHP system connected to a test room with 16.24 m² floor area in Fırat University, Elazığ (38.41°N, 39.14°E), Turkey, was designed and constructed. The heating and cooling loads of the test room were 2.5 and 3.1 kW at design conditions, respectively. The system was commissioned in November 2002 and the performance tests have been carried out since then. The average performance coefficients of the system (COPS) for horizontal ground heat exchanger (GHE) in the different trenches, at 1 and 2 m depths, were obtained to be 2.92 and 3.2, respectively. Experimental performances were performed to verify the results from the ANFIS approach. In order to achieve the optimal result, several computer simulations have been carried out with different membership functions and various number of membership functions. The most suitable membership function and number of membership functions are found as Gauss and 2, respectively. For this number level, after the training, it is found that root-mean squared (RMS) value is 0.0047, and absolute fraction of variance (R²) value is 0.9999 and coefficient of variation in percent (cov) value is 0.1363. This paper shows that the values predicted with the ANFIS, especially with the hybrid learning algorithm, can be used to predict the performance of the GCHP system quite accurately.     

Two-phase pressure drop of R-410A in horizontal smooth minichannels

Convective boiling pressure drop experiments were performed in horizontal minichannels with a binary mixture refrigerant, R-410A. The test section was made of stainless steel tubes with inner diameters of 1.5 mm and 3.0 mm and with lengths of 1500 mm and 3000 mm, respectively. This test section was uniformly heated by applying electric current directly to the tubes. Experiments were performed at inlet saturation temperature of 10 °C, mass flux ranges from 300 to 600 kg m⁻² s⁻¹ and heat flux ranges from 10 to 40 kW m⁻². The current study showed the significant effect of mass flux and tube diameter on pressure drop. The experimental results were compared against 15 two-phase pressure drop prediction methods. The homogeneous model predicted well the experimental pressure drop, generally. A new pressure drop prediction method based on the Lockhart–Martinelli method was developed with 4.02% mean deviation.     

Compressor driven metal hydride cooling systems—mathematical model and operating characteristics

A detailed mathematical model of a compressor driven metal hydride (CDMH) cooling system is presented. The model takes into account the transient characteristics of the coupled metal hydride reactors with external fins, reciprocating compressor and conditioned space. The model can be used for predicting the system performance during the initial pull-down as well as the stable cycling periods. Results are obtained for a 1 kW capacity air conditioning system using optimized reactors packed with Zr_0.9Ti_0.1Cr_0.55Fe_1.45 hydride. The COP's obtained are in the range of 1.7–2.2 depending on operating conditions while the specific cooling power obtained is about 600 kJ kg alloy⁻¹ h⁻¹. It is shown that in order to make these systems commercially viable it is important to select suitable materials with fast reaction kinetics. Since, system performance parameters can be predicted more effectively, the model can be used for design of optimized systems.     

Split heat pipe type compound adsorption ice making test unit for fishing boats

In order to settle the problem of the corrosion between sea water and the steel adsorber for ammonia system, a split heat pipe type adsorption ice making test unit, which use compound adsorbent of CaCl₂ and activated carbon to improve the adsorption performance, is designed and constructed. For this test unit there is mass recovery function between two beds and the CaCl₂ in compound adsorbent per bed is 1.88 kg, and there is only one pump for the whole heating and cooling phase for adsorbers. Performances of this system are tested; the lowest evaporating temperature is as low as −42 °C. At the evaporating temperature of −35 and −25 °C, the cooling powers are 0.89 and 1.18 kW, respectively. At the evaporating temperature of −15 °C, its average cooling power is 1.37 kW, which corresponds coefficient of performance of refrigeration COP=0.41 and specific cooling power per kilogram CaCl₂ of each adsorber SCP=731 W kg⁻¹. The mass recovery process has improved SCP and COP for the system by 15.5 and 24.1%, respectively. Heat transfer performance is also improved by the split heat pipe construction; the average heat transfer coefficient for a whole cycle is 155.8 W m⁻² °C⁻¹.     

Spray absorbers in absorption systems using lithium nitrate–ammonia solution

In the present work, the spray absorption method is studied for the absorption of ammonia refrigerant vapour by lithium nitrate–ammonia solutions. Mass transfer coefficients attainable using the spray absorption method are estimated. In this study the low-pressure absorber of a double-stage absorption refrigeration system is considered. Results show that the mass transferred is maximum (about 60% of the total) during the deceleration period of the drops. This period represents about 13.4% of the time required to reach the equilibrium state at the end of the absorption chamber. The results show that a time-average mass transfer coefficient equal to k_m=18.6×10⁻⁵ m s⁻¹ may be attained.     

Experimental studies on the evaporative heat transfer and pressure drop of CO2 in smooth and micro-fin tubes of the diameters of 5 and 9.52 mm

Carbon dioxide among natural refrigerants has gained a considerable attention as an alternative refrigerant due to its excellent thermophysical properties. In-tube evaporation heat transfer characteristics of carbon dioxide were experimentally investigated and analyzed as a function of evaporating temperature, mass flux, heat flux and tube geometry. Heat transfer coefficient data during evaporation process of carbon dioxide were measured for 5 m long smooth and micro-fin tubes with outer diameters of 5 and 9.52 mm. The tests were conducted at mass fluxes of from 212 to 656 kg m⁻² s⁻¹, saturation temperatures of from 0 to 20 °C and heat fluxes of from 6 to 20 kW m⁻². The difference of heat transfer characteristics between smooth and micro-fin tubes and the effect of mass flux, heat flux, and evaporation temperature on enhancement factor (EF) and penalty factor (PF) were presented. Average evaporation heat transfer coefficients for a micro-fin tube were approximately 150–200% for 9.52 mm OD tube and 170–210% for 5 mm OD tube higher than those for the smooth tube at the same test conditions. The effect of pressure drop expressed by measured penalty factor of 1.2–1.35 was smaller than that of heat transfer enhancement.     

Heat transfer of oil-contaminated HFC134a in a horizontal evaporator

The introduction of chlorine-free refrigerants to the market requires experimental investigations of their behaviour in heat pumps and refrigerators. One particular area of interest is the effect of the new oils on the heat transfer in evaporators and condensers. Oil can either increase or decrease the heat transfer coefficient. This paper presents the results from an experimental investigation of the effect of three different ester-based oils on the heat transfer of HFC134a in a horizontal evaporator. The tests were carried out at heat fluxes between 2 and 8 kW m⁻² (corresponding to mass fluxes between approximately 40 and 170 kg s⁻¹ m⁻²). The evaporation temperature was varied from−10 to +10°C. The global oil concentration ranged from 0 to 4.5 mass percentage based on the total liquid flow. The heat transfer coefficient decreased in most of the cases. The results indicate that the decrease seems to depend on the viscosity of the oil. The decrease can fairly well be estimated with the correlation for pure refrigerants by Shah if the viscosity of the mixture is used in the calculations. The data for the oil-contaminated refrigerant also agree well with data for pure refrigerants in a plot of α_tp/α_lo^* versus the inverse Martinelli-Lockhart parameter when α_lo^* is calculated with a modified Dittus-Boelter correlation and the mixture viscosity is used in the calculations. The heat transfer is found to increase when introducing oil in the special cases where the flow rate is low and the viscosity is low (oil A, 2 and 4 kW m⁻² oil B, 6kW m⁻² at +10°C). This is most likely due to surface tension effects. It has been suggested that the increased surface tension leads to a better tube wetting and thus an increased heat transfer.     

Experimental performance analysis and modelling of liquid desiccant cooling systems for air conditioning in residential buildings

The paper presents a new desiccant cooling cycle to be integrated in residential mechanical ventilation systems. The process shifts the air treatment completely to the return air side, so that the supply air can be cooled by a heat exchanger. Purely sensible cooling is an essential requirement for residential buildings with no maintenance guarantee for supply air humidifiers. As the cooling power is generated on the exhaust air side, the dehumidification process needs to be highly efficient to provide low supply air temperatures. Solid rotating desiccant wheels have been experimentally compared with liquid sorption systems using contact matrix absorbers and cross flow heat exchangers. The best dehumidification performance at no temperature increase was obtained in an evaporatively cooled heat exchanger with sprayed lithium chloride solution. Up to 7 g kg⁻¹ dehumidification could be reached in an isothermal process, although the surface wetting of the first prototype was low. The process then provides inlet air conditions below 20 °C for the summer design conditions of 32 °C, 40% relative humidity. With air volume flow rates of 200 m³ h⁻¹ the system can provide 886 W of cooling power.A theoretical model for both the contact absorber and the cross flow system has been developed and validated against experimental data for a wide range of operating conditions. A simulation study identified the optimisation potential of the system, if for example the surface wetting of the liquid desiccant can be improved.     

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.