Experimental and theoretical investigation of an innovative evaporative condenser for residential refrigerator

In this study, an innovative, evaporative condenser for residential refrigerator was introduced. A vapor compression cycle incorporating the proposed evaporative condenser was tested to evaluate the cycle performance. To allow for evaporative cooling, sheets of cloth were wrapped around condenser to suck the water from a water basin by capillary effect. The thermal properties at the different points of the refrigeration cycle were measured for typical operating conditions. The experimental results showed that the condenser temperature increases 0.45 °C for each degree increase in evaporator temperature when the air velocity is 2.5 m/s, and the ambient condition is 29 °C and the relative humidity is 37.5%. Meanwhile, the condenser temperature increase is 0.88 °C in the case of air velocity 1.1 m/s and ambient conditions of 31 °C and relative humidity of 47.1%. A theoretical model for the evaporative condenser was developed, and validated by experimental results. The theoretical model showed that the evaporative condenser can operate at a condensing temperature of 20 C lower than that of the air-cooled condenser for heat flux of 150 W/m², and at air velocity 3 m/s. The effect of the different parameters on the condenser temperature was studied too.     

Numerical investigation of a two-stage air-cooled absorption refrigeration system for solar cooling: Cycle analysis and absorption cooling performances

Two-stage air-cooled ammonia–water absorption refrigeration system could make good use of low-grade solar thermal energy to produce cooling effect. The system simulation results show that thermal COP is 0.34 and electrical COP is 26 under a typical summer condition with 85 °C hot water supplied from solar collector. System performances under variable working conditions are also analyzed. Circular finned tube bundles are selected to build the air-cooled equipment. The condenser should be arranged in the front to get an optimum system performance. The mathematical model of the two-stage air-cooled absorber considering simultaneous heat and mass transfer processes is developed. Low pressure absorber should be arranged in front of middle pressure absorber to minimize the absorption length. Configuration of the air-cooled equipment is suggested for a 5 kW cooling capacity system. Temperature and concentration profiles along the finned tube length show that mass transfer resistance mainly exists in liquid phase while heat transfer resistance mainly exists in cooling air side. The impacts on system refrigeration capacities related to absorption behaviors under variable working conditions are also investigated. Both cycle analysis and absorption performances show that two-stage air-cooled ammonia–water absorption chiller is technically feasible in practical solar cooling applications.     

Heat transfer during condensation of moving steam in a narrow channel

The paper presents the results of experimental investigation of heat transfer and hydrodynamics during condensation of moving steam in a narrow channel of square cross-section 2 mm × 2 mm. The channel had a serpentine shape, the channel length was 660 mm. An experimental cell simulated conditions of heat transfer in the condenser of loop heat pipes. The steam velocity at the channel inlet ranged from 13 to 52 m/s, the pressure was 1 atm. The temperature of the cooling water varied from 70 to 95 °C. The annular flow pattern was noted in the whole range of the regime parameters. There was a clear boundary between the condensation zone and the zone occupied by the condensed phase downstream. Temperature has measured along the channel, and the heat-transfer coefficients have been determined. The coefficient values varied from 10,000 to 55,000 W/K m² depending on the steam velocity at the channel inlet and the cooling temperature. The efficiency of the condenser – heat exchanger has been investigated.     

An experimental investigation of steam ejector refrigeration system powered by extra low temperature heat source

A steam ejector refrigeration system is a low capital cost solution for utilizing industrial waste heat or solar energy. When the heat source temperature is lower than 80 °C, the utilization of the thermal energy from such a low-temperature heat source can be a considerable challenge. In this investigation, an experimental prototype for the steam ejector refrigeration system was designed and manufactured, which can operate using extra low-temperature heat source below 80 °C. The effects of the operation temperature, the nozzle exit position (NXP) and the diameter of the constant area section on the working performance of the steam ejector were investigated at generating temperatures ranging from 40 °C to 70 °C. Three ejectors with a same de Laval nozzle for the primary nozzle and three different constant-area sections were designed and fabricated. The experimental results show that a steam ejector can function for a certain configuration size of the steam ejector with a generating temperature ranging from 40 °C to 70 °C and an evaporating temperature of 10 °C. For a given NXP, the system COP and cooling capacity of the steam ejector decreased until inoperative as the diameter of the constant area section reduced. The results of this investigation provided a good solution for the refrigeration application of the steam ejector refrigeration system powered by an extra low-temperature heat source.     

Experimental study of a solid adsorption cooling system using flat-tube heat exchangers as adsorption bed

In this paper, a solid adsorption cooling system with silica gel as the adsorbent and water as the adsorbate was experimentally studied. To reduce the manufacturing costs and simplify the construction of the adsorption chiller, a vacuum tank was designed to contain the adsorption bed and evaporator/condenser. Flat-tube type heat exchangers were used for adsorption beds in order to increase the heat transfer area and improve the heat transfer ability between the adsorbent and heat exchanger fins. Under the standard test conditions of 80 °C hot water, 30 °C cooling water, and 14 °C chilled water inlet temperatures, a cooling power of 4.3 kW and a coefficient of performance (COP) for cooling of 0.45 can be achieved. It has provided a specific cooling power (SCP) of about 176 W/(kg adsorbent). With lower hot water flow rates, a higher COP of 0.53 can be achieved.     

Natural convection heat transfer of molten salts around a vertically aligned horizontal cylinder set

The heat transfer from the vertical arrays of a set of equally spaced cylinders in molten salts is studied numerically to obtain the laminar natural convection heat transfer mechanism of molten salts around a vertically aligned horizontal cylinder set. Simulations are performed for arrays of 2–10 horizontal cylinders at a Rayleigh number based on a cylinder diameter between 2 × 10³ and 5 × 10⁵. Results show that the natural convective heat transfer of molten salts from the bottom cylinder of the array remains the same as that from a single cylinder. By contrast, the downstream cylinders may either be enhanced or reduced mainly depending on their location in the array and on the tube spacing. Heat transfer dimensionless correlating equations are proposed for any individual cylinder in the two vertically aligned horizontal cylinders. The heat transfer mechanism from the horizontal cylinders set in a vertical array is also simulated, and the results show that cylinder spacing can influence the average heat transfer rate around the whole tube array. Thus, in real applications, adjusting the cylinder spacing better enhances the average heat transfer from the whole tube array.     

Performance assessment of a direct expansion air conditioner working with R407C as an R22 alternative

This paper presents an experimental investigation of a direct expansion air conditioner working with R407C as an R22 alternative. Experiments are conducted on a vapor compression refrigeration system using air as a secondary fluid through both the evaporator and the condenser. The influences of the evaporator air inlet temperature (20–32 °C), the evaporator air flow rate (250–700 m³/h) and the evaporator air humidity ratio (9 and 14.5 g_wv/kg_a) at the condenser air temperature and volume flow rate of 35 °C and 850 m³/h, respectively on the system performance are investigated. Experimental results revealed that the evaporator air inlet temperature has pronounced effects on the air exit temperatures, pressures of the evaporator and the condenser, cooling capacity, condenser heat load, compressor pressure ratio and the COP of both refrigerants at humidity ratios of 9 and 14.5 g_wv/kg_a. Significant effects of the evaporator air flow rate are also gathered on the preceding parameters at the same values of mentioned-humidity ratios. The best performance, in terms of operating parameters as well as COP, can be accomplished using R22 compared to R407C. The inlet humidity ratio affects dramatically the performance of vapor compression system using R22 and R407C. The raising up humidity ratio from 9 to 14.5 g_wv/kg_a leads to an augmentation in the average cooling capacity by 29.4% and 38.5% and an enhancement in the average COP by 30% and 24.1% for R22 and R407C, respectively.     

Optimum temperatures in a shell and tube condenser with respect to exergy

This paper focuses on evaluation of the optimum cooling water temperature during condensation of saturated water vapor within a shell and tube condenser, through minimization of exergy destruction. First, the relevant exergy destruction is mathematically derived and expressed as a function of operating temperatures and mass flow rates of both vapor and coolant. The optimization problem is defined subject to condensation of the entire vapor mass flow and it is solved based on the sequential quadratic programming (SQP) method. The optimization results are obtained at two different condensation temperatures of 46 °C and 54 °C for an industrial condenser. As the upstream steam mass flow rates increase, the optimal inlet cooling water temperature and exergy efficiency decrease, whereas exergy destruction increases. However, the results are higher for optimum values at a condensation temperature of 54 °C, compared to those when the condensation temperature is 46 °C. For example, when the steam mass flow rate is 1 kg/s and the condensation temperature increases from 46 °C to 54 °C, the optimal upstream coolant temperature increases from 16.78 °C to 25.17 °C. Also, assuming an ambient temperature of 15 °C, the exergy destruction decreases from 172.5 kW to 164.6 kW. A linear dependence of exergy efficiency on dimensionless temperature is described in terms of the ratio of the temperature difference between the inlet cooling water and the environment, to the temperature difference between condensation and environment.     

Prediction of the maximum heat transfer capability of two-phase heat spreaders – Experimental validation

The present paper is devoted to an experimental study to determine the thermal behaviour of a two-phase heat spreader (TPHS) with micro-grooves. The proposed application is the cooling of fuel cell systems. This TPHS aims at reducing the volume of actual cooling systems and to homogenize the temperature in the hearth of fuel cells. The TPHS is flat with a wide evaporating area (190 × 90 mm²) compared to the condenser area (30 × 90 mm²). It has been tested with three working fluids: water, methanol and n-pentane. Experimental results obtained with methanol show a temperature difference lower than 1.6 K on the entire evaporator area for a heat transfer rate equal to 85 W and a working temperature equal to 70 °C. The TPHS has been tested in both horizontal and vertical favourable orientation (thermosyphon orientation). The temperature field is similar in both cases for heat transfer rates lower than 155 W. In horizontal orientation, a confocal microscope is used to measure the meniscus curvature radius along the grooves. A two-phase flow model allowing the calculation of the meniscus radius, the liquid and vapour pressures and the liquid and vapour velocities along the TPHS is developed. The comparison between experimental and model results shows the good ability of the numerical model to predict the meniscus curvature radii from which the maximum heat transfer capability of the TPHS is depending.     

A study on the performance enhancement of heat pump using electric heater under the frosting condition: Heat pump under frosting condition

The purpose of the present study is to enhance the heating capacity and increase COP under the frosting condition during heating operation of small capacity air-to-air heat pump. We applied an electric heater in front of outdoor unit of heat pump instead of indoor unit as usual. When the outdoor temperature is 2 °C/1 °C (DB/WB), the present heat pump turns on the electric heater in outdoor unit. The heating capacity increases 38.0% and COP increases 57.0% in comparison with those of conventional heat pump. When the outdoor temperature is 4 °C/2 °C (DB/WB), the electric heater is in ON/OFF mode according to the temperature of the evaporator. The heating capacity increases 9.1% and COP increases 71.1% in comparison with those of conventional heat pump.     

Influence of hydrodynamic instability on the heat transfer coefficient during condensation of R134a and R404A refrigerants in pipe mini-channels

This paper presents the results of an investigation of the influence of hydrodynamic instabilities on heat transfer intensity during the condensation of R134a and R404A refrigerants in pipe mini-channels. The heat transfer coefficient h is a measure of the effectiveness of the condensation process. It is particularly important to determine the value of the coefficient in the two-phase condensation area in a compact condenser. In other condenser areas (i.e., precooling of superheated vapor and subcooling of condensate), the heat efficiency is substantially smaller. Hydrodynamic instabilities of a periodic nature have an influence on size changes in these areas. A decrease in the heat transfer coefficient h in the two-phase area results in decreased intensity of the heat removal process in the whole condenser.The experimental investigations were based on the condensation of R134a and R404A refrigerants in horizontal pipe mini-channels with internal diameters of d = 0.64; 0.90; 1.40; 1.44; 1.92; 2.30 and 3.30 mm. Disturbances of the condensation process were induced with a periodic stop and a repetition of the flow of the refrigerant.In the range of frequencies, f = 0.25–5 Hz, of the periodically generated disturbances, an unfavorable influence on the intensity of the heat transfer during the condensation process in pipe mini-channels was identified. The reduction in the intensity of the heat transfer during the condensation process, which was induced with hydrodynamic instabilities, was presented in the form of the dependence of the heat transfer coefficient h on the vapor quality x and the frequencies f of the disturbances.The influence of the refrigerant, the diameter of the mini-channels and the frequency f on the damping phenomenon of the periodical disturbances in the pipe mini-channels was identified.     

Experimental results on operating parameters influence for an adsorption refrigerator

A continuous heat recovery adsorption refrigerator using activated carbon-methanol has been developed. In this system, the heat source to drive the adsorption system can be controlled at a temperature from 60 °C to 110 °C, and the evaporating temperature can also be controlled at any requested value from 0 °C to 15 °C. To realize the operation performance of the system, many sensors of temperature, pressure and flow rate are installed in the adsorbers, the condenser and the evaporator. A lot of experiments have been completed in different operation conditions. Thus, by means of the experimental data, influences of the operating parameters, such as heat source temperature, evaporating temperature, cooling water temperature, cycle time and flow rate of throttling valve and so on, on p-t-x diagram of the cycle, specific cooling power (SCP) and coefficient of performance (COP) have been asserted. And causes of the influence are also analyzed. A series of conclusions are obtained.     

Cooling of two smooth cylinders in row by a slot jet of air with low turbulence

According to the current literature on the cooling of two cylinders in row, by a uniform flow of air, the first cylinder is always a heat transfer promoter versus the second one. The aim of the present paper is to summarize the state of art of the literature on the cooling of two cylinders in row by a slot jet of air. Additional experiments are carried on in order to investigate the possible application of jet cooling to heat transfer apparatuses, including electronics, in order to study the positions of the two cylinders in row which realize the same heat transfer on each cylinder. In the experiments a slot jet of air with low turbulence is employed with a slot height, S, equal to the impinged cylinder diameter, D, i.e. D/S = 1.0. The first cylinder is set at two distances H from the slot exit, H/S = 4 and 6, while the distance of the second cylinder from the first one, L, is variable from L/S = 2–11. The Reynolds number, Re, defined with the cylinder diameter D, spans in the range Re = 11,000–22,200. If the first cylinder is set at the dimensionless distance from the slot exit which realizes the maximum mean heat transfer on the first cylinder, i.e. H/S = 6, the second one has generally a lower mean Nusselt number. The only exception is when the second cylinder is set at the dimensionless distance L/S = 4 and the Reynolds number is at the maximum value experimented, i.e. Re = 22,200. If the first cylinder is set at the dimensionless distance H/S = 4 the mean Nusselt number on the second cylinder is greater if its distance from the first one is in the range L/S = 3.5–7 for Re = 14,300–22,200. The first cylinder acts as a heat transfer promoter, as happens in uniform flow, only for Re = 22,200.     

Exergoeconomic analysis of condenser type heat exchangers

In this study, an exergoeconomic analysis of condenser type parallel flow heat exchangers is presented. Exergy losses of the heat exchanger and investment and operation expenses related to this are determined with functions of steam mass flow rate and water exit temperature at constant values of thermal power of the heat exchanger at 75240 W, cold water mass flow rate and temperature. The inlet temperature of water is 18 °C and exit temperatures of water are varied from 25 °C to 36 °C. The values of temperature and pressure of saturated steam in the condenser are given to be T_con=47 ° C and P_con=10.53 kPa. Constant environment conditions are assumed. Annual operation hour and unit price of electrical energy are taken into account for determination of the annual operation expenses. Investment expenses are obtained according to the variation of heat capacity rate and logarithmic mean temperature difference and also heat exchanger dimension determined for each situation. The present analysis is hoped to be useful in determining the effective parameters for the most appropriate exergy losses together with operating conditions and in finding the optimum working points for the condenser type heat exchangers.     

Effect of rotating cylinder on heat transfer in a square enclosure filled with nanofluids

Convective heat transfer in a differentially heated square enclosure with an inner rotating cylinder is studied theoretically. The free space between the cylinder and the enclosure walls is filled with water–Ag, water–Cu, water–Al₂O₃ or water–TiO₂ nanofluids. The governing equations are formulated for velocity, pressure and temperature formulation and are modeled in COMSOL, a partial differential equation (PDE) solver based on the Galerkin finite element method (GFEM). The governing parameters considered are the solid volume fraction, 0.0 ? ? ? 0.05, the cylinder radius, 0 ? R ? 0.3 and the angular rotational velocity, ?1000 ? Ω ? 1000. The results are presented to show the effect of these parameters on the heat transfer and fluid flow characteristics. It is found that the strength of the flow circulation is much stronger for a higher nanoparticle concentration, a better thermal conductivity value and a smaller cylinder with a faster, negative rotation. The maximum heat transfer are obtained at a high nanoparticle concentration with a good conductivity value, a slow positive rotation and a moderate cylinder size located in the center of the enclosure.     

Theoretical analysis and experimental research on transcritical CO2 two stage compression cycle with two gas coolers (TSCC + TG) and the cycle with intercooler (TSCC + IC)

As one of the natural refrigerants, CO₂ is a potential substitute for synthesized refrigerants with favorable environmental properties. In order to improve the performance of the CO₂ transcritical compression cycle, the performance of the two stage compression cycle with two gas coolers (TSCC + TG) and the two stage compression cycle with intercooler (TSCC + IC) were analyzed, respectively. Under the given calculation condition, the optimum intermediate pressure of the cycle TSCC + TG and the TSCC + IC are 7.09 MPa and 5.89 MPa, and the maximal COP are 2.77 and 3.08, respectively. Range of the given evaporating temperature and outlet temperature of gas cooler, the experimental testing shows that the performance of cycle TSCC + IC are 11.88% and 10.87% better than that of the cycle TSCC + TG, respectively. Range of the given inlet temperature and cooling water volume flow of gas cooler, the refrigeration COP (COP_c) and heat COP (COP_h) of the cycle TSCC + IC are average 10.97% and 4.39% higher than that of the cycle TSCC + TG. Range of the given inlet temperature and chilled water volume flow of evaporator, the refrigeration COP (COP_c) and heat COP (COP_h) of the cycle TSCC + IC are average 10.71% and 3.67% higher than that of the cycle TSCC + TG, respectively. The error between theoretical calculation and experimental testing is not exceeds 20%.     

Experimental investigation of cryogenic oscillating heat pipes

A novel cryogenic heat pipe, oscillating heat pipe (OHP), which consists of an 4 × 18.5 cm evaporator, a 6 × 18.5 cm condenser, and 10 cm length of adiabatic section, has been developed and experimental characterization conducted. Experimental results show that the maximum heat transport capability of the OHP reached 380 W with average temperature difference of 49 °C between the evaporator and condenser when the cryogenic OHP was charged with liquid nitrogen at 48% (v/v) and operated in a horizontal direction. The thermal resistance decreased from 0.256 to 0.112 while the heat load increased from 22.5 to 321.8 W. When the OHP was operated at a steady state and an incremental heat load was added to it, the OHP operation changed from a steady state to an unsteady state until a new steady state was reached. This process can be divided into three regions: (I) unsteady state; (II) transient state; and (III) new steady state. In the steady state, the amplitude of temperature change in the evaporator is smaller than that of the condenser while the temperature response keeps the same frequency both in the evaporator and the condenser. The experimental results also showed that the amplitude of temperature difference between the evaporator and the condenser decreased when the heat load increased.     

In-tube passive heat transfer enhancement in the process industry

Enhanced heat transfer surfaces are used in heat exchangers to improve performance and to decrease system volume and cost. In-tube heat transfer enhancement usually takes the form of either micro-fin tubes (of the helical micro-fin or herringbone varieties), or of helical wire inserts. Despite a substantial increase in heat transfer, these devices also cause non-negligible pressure drops.By making use of well-proven flow pattern maps for smooth tubes and the new ones for smooth and enhanced tubes, it is shown from the refrigerant condensation data that flow patterns have a strong influence on heat transfer and pressure drop. This is done for data obtained from in-tube condensation experiments for mass fluxes ranging from 300 to 800 kg/m² s at a saturation temperature of 40 °C, for refrigerants R-22, R-134a, and R-407C. The flow regimes, pressure drops, heat transfer coefficients, and the overall performance of three different tubes, namely a smooth-, 18° helical micro-fin-, and a herringbone micro-fin tube (each having a nominal diameter of 9.51 mm), are presented and compared to the performance of smooth tubes with helical wire inserts (with pitches of 5 mm, 7.77 mm and 11 mm corresponding to helical angles of 78.2°, 72°, and 65.3°, respectively).     

Experimental study of conjugate heat transfer within a bottom heated vertical concentric cylindrical enclosure

In this research work an experimental study of conjugate heat transfer within an air filled bottom-heated vertical enclosure is conducted. The enclosure consists of two concentric cylinders with inner cylinder being shorter and open at the top. The study is important with respect to the centrifuge machine used in the process industry. Eighteen different experiments are performed by varying the bottom disc central temperature between 353 and 433 K, using three different materials (aluminum, mild steel and stainless steel) of the inner cylinder and two different diameter outer cylinders of mild steel. This study unfolds the temperature, material and geometric effects of bottom disc, inner cylinder and outer cylinder respectively on thermal convection in the enclosure. Generally, a uniform temperature is required in such enclosures. A more uniform axial and radial temperature is observed in the enclosure by using aluminum inner cylinder within a temperature range of 353–433 K of the bottom disc and using two different diameter outer cylinders. It is observed that the maximum temperature in the enclosure is lowest for aluminum inner cylinder and higher for mild steel and highest for stainless steel. The heat balance and non-dimensional analysis of the enclosure are carried out and discussed critically.     

Heat transfer performance in 3D internally finned heat pipe

An experimental study of heat transfer performance in 3D internally finned steel-water heat pipe was carried out in this project. All the main parameters that can significantly influence the heat transfer performance of heat pipe, such as working temperature, heat flux, inclination angle, working fluid fill ratio (defined by the evaporation volume), have been examined. Within the experimental conditions (working temperature 40 °C–95 °C, heat flux 5.0 kw/m²–40 kw/m², inclination angle 2–90°), the evaporation and condensation heat transfer coefficients in 3D internally finned heat pipe are found to be increased by 50–100% and 100–200%, respectively, as compared to the smooth gravity-assisted heat pipe under the same conditions. Therefore, it is concluded that the special structures of 3D-fins on the inner wall can significantly reduce the internal thermal resistance of heat pipe and then greatly enhance its heat transfer performance.