An experimental analysis of the effect of refrigerant charge level on an automotive refrigeration system

The performance of an automotive refrigeration system is dependent on the refrigerant charge level. Due to inevitable leaks in the system, the amount of refrigerant will decrease over time and thus ultimately reduce the system's performance. A reduction in the amount of refrigerant charge results in excessive compressor cycling, a lower condenser pressure, a higher refrigeration temperature, and an increase in the amount of superheat. This paper identifies and quantifies the individual component losses in an automotive refrigeration system as a function of the refrigerant charge level. A second law analysis, based on nondimensional entropy generation, is carried out to quantify the thermodynamic losses. A passenger vehicle with a cycling-clutch, orifice tube refrigeration system was instrumented to measure various temperatures and pressures, and relative humidity. The data were collected at idle conditions. Thermodynamic equations, which are used to determine the system's thermal performance, are presented. The system's second law efficiency increases 26 % as the amount of refrigerant charge decreases by 44 %. Also the individual component losses are quantified as a function of the refrigerant charge level. The compressor and the condenser losses account for the largest percentage of the total losses, and are of similar magnitude. The evaporator–accumulator and the orifice tube losses account for a smaller percentage of the total losses, and are also of similar magnitude. With a reduction in the refrigerant charge level of 44 %, the losses in the compressor, the condenser, the evaporator–accumulator, and the orifice tube decrease 13 %, 8 %, 10 %, and 33 %, respectively.     

Study on performance of a heat pump water heater using suction stream liquid injection

Liquid refrigerant injection into a suction line is an effective and practical method to reduce the discharge temperature when a scroll compressor operates at high compression ratios. In the present study, correlations among the compressor suction temperature, discharge temperature, heat pump heating capacity, power consumption, coefficient of performance (COP) and the quantity of suction liquid injection are established. The paper presents experimental analysis and a comparison with calculated results of the heat pump water heater (HPWH) performance with suction liquid injection in different conditions. It is found that the suction liquid injection explicitly lowers the discharge temperature of the compressor and the heating capacity of the unit, but the power consumption increases with COP decreasing. In addition, the highest injection ratio must be controlled fewer than 5%. The suction liquid injection has a better effect on the HPWH at the temperature ranging from ?15 °C to 20 °C. Within this temperature range, the 5% ratio suction liquid injection decreases the discharge temperature of the compressor by 10 °C, while the heating capacity of the HPWH decreases by less than 5%, power consumption increases by less than 1.5%, and COP decreases by less than 7%.     

Influence of oil on R-410A two-phase frictional pressure drop in a small U-type wavy tube

This study presents single-phase and two-phase pressure drop data with oil concentration C = 0, 1, 3 and 5% in a copper wavy tube having an inner diameter of 3.25 mm and a curvature radius of 6.35 mm. The ratio of frictional factor between U-bend in wavy tube and straight tube (f_C/f_S) is about 1.5 to 2.5 for Re = 2500 ∼ 25000. The effect of secondary flow is very crucial in the U-bend that it increases the pressure drop considerably. However, the effect of oil concentration on friction factor is negligible provided the properties are based on mixture. The ratio between two-phase pressure gradients of U-bend and straight tube is about 3. This ratio is increased with oil concentration and vapor quality. The oil effect on two-phase pressure drop is especially pronounced at high vapor quality because the effective oil concentration in liquid mixture is increased with vapor quality. The frictional two-phase multiplier for straight tube can be fairly correlated by using the Chisholm correlation. A modified two-phase friction factor based on the Geary correlation is also utilized to predict the frictional two-phase pressure gradient in U-bend. The predictions give a good agreement to the present oil–refrigerant data with a mean deviation of 12.92%.     

Effect of nanoparticle size on nucleate pool boiling heat transfer of refrigerant/oil mixture with nanoparticles

Effect of nanoparticle size on nucleate pool boiling heat transfer of refrigerant/oil mixture with nanoparticles was investigated experimentally. For the preparation of the test fluid, refrigerant R113, ester oil VG68, and Cu nanoparticles with three different average diameters of 20, 50 and 80 nm were used. Experimental conditions include a saturation pressure of 101.3 kPa, heat fluxes from 10 to 80 kW m^?2, nanoparticle concentrations in the nanoparticles/oil suspension from 0 to 30 wt%, and nanoparticles/oil suspension concentrations from 0 to 5 wt%. The experimental results indicate that the nucleate pool boiling heat transfer coefficient of R113/oil mixture with Cu nanoparticles is enhanced by a maximum of 23.8% with the decrease of nanoparticle size from 80 to 20 nm under the present experimental conditions, and the enhancement increases with the decrease of nanoparticles/oil suspension concentration or the increase of nanoparticles concentrations in the nanoparticles/oil suspension. A general nucleate pool boiling heat transfer coefficient correlation for refrigerant/oil mixture with nanoparticles is proposed, and it agrees with 93% of the existing experimental data of refrigerant/oil mixture with nanoparticles within a deviation of ±20%.     

Characterization of a scroll compressor under extended operating conditions

Refrigeration and air-conditioning compressors are designed to work under well-defined conditions. In some applications it is interesting to observe their performances beyond these conditions, for example in the case of a high temperature two-stage heat pump or of a cooling system working at high temperature.In this study a compressor is characterized experimentally with refrigerant R134a and through 118 tests at condensing pressures varying from 8.6 up to 40.4 bar (t_sat = 33.9 °C to t_sat = 100.8 °C) and evaporating pressures varying from 1.6 up to 17.8 bar (t_sat = ?15.6 °C to t_sat = 62.4 °C). Under these conditions the compressor motor was pushed at its maximal current in several tests.This compressor’s performance is mainly characterized by its isentropic and volumetric efficiencies. It presents a maximal isentropic efficiency of 72%, corresponding to a pressure ratio of around 2.5–2.6. The volumetric efficiency decreases linearly from almost 1.0 (for a pressure ratio of 1.3) to 0.83 (for a pressure ratio of 9.7). A slight degradation of the isentropic and volumetric efficiencies is observed when the compressor supply and exhaust pressures are increased for a given pressure ratio; this could be due to an internal leakage.The compressor tests are used to identify the six parameters of a semi-empirical simulation model. After parameter identification, experimental and simulated results are in very good agreement, except for some points at high compressor power where the compressor is pushed at its maximal current.     

Formulating and optimizing a combustion pathways for oil shale and its semi-coke

Enhanced technologies from oil recovery to unconventional fuels - oil shale, oil sands and extra-heavy oil – have in common complex chemical reactions processes. This paper is about the formulation and optimization of the chemical mechanism especially in oil shale and semi-coke combustion. The Levenberg–Marquardt algorithm was used to minimize the error between estimated values and the thermogravimetric data for combustion mechanisms of 4-steps and 3-steps proposed for the oil shale and its semi-coke respectively. The kinetic parameters such as reaction order, pre-exponential factor, activation energy and stoichiometric coefficients that affect drying, pyrolysis, oxidation and decarbonation reactions were estimated with success. The values of activation energies were 54–67 kJ mol^?1 for oil shale drying, 62–65 kJ mol^?1 for pyrolysis reaction, up to 100 kJ mol^?1 for Fixed Carbon (FC) oxidation reaction, and 162–418 kJ mol^?1 for decarbonation reaction. Regarding to the semi-coke combustion, the activation energies were 33 kJ mol^?1 for drying reaction, 211 kJ mol^?1 for oxidation reaction and 291 kJ mol^?1 for decarbonation reaction. The chemical reactions suggest reaction order superior to one, except to the decarbonation reaction at 3 K min^?1. Considering the estimated parameters, as well as a heating rate at 3 K min^?1, an oil shale containing about 20 wt.% of organic matter and 34.6 wt.% of CaCO₃, the species mass fractions formed during combustion process were 3.4 wt.% of FC, 10.6 wt.% of Oil, 3.3 wt.% of HC and 1.8 wt.% of CO. The fraction of CO₂ formed accounts a total of 21.6 wt.%. For a semi-coke containing 3.4 wt.% of FC and 40.6 wt.% of CaCO₃, its combustion formed 2.1 wt.% of CO. The CO₂ fraction from oxidation and decarbonation reactions accounts 10.2 wt.%, considering that the stoichiometric mass coefficient γ = 0.75 in decarbonation reaction.     

Experimental evaluation on the oil influence in capillary tubes

This work presents the results of an experimental study on pure refrigerant R-134a and refrigerant–oil mixtures flowing through capillary tubes in order to analyse the oil influence in component performance. Tests were carried out for capillary tubes internal diameters of 0.69 mm and 0.82 mm, condensing temperatures ranging from 40 °C to 50 °C, and subcooling degrees between 3 °C and 12 °C. Pure refrigerant flow measurements were compared to those for refrigerant–oil mixtures with oil concentrations of 1.0% and 3.0%.     

Influence of the superheat associated to a semihermetic compressor of a transcritical CO2 refrigeration plant

This work evaluates, from an energetic point of view, the effects of the superheat caused in the refrigerant by the electric motor cooling (SH_SC) in a semihermetic compressor installed in an experimental refrigerating plant, which operates with CO₂ as the working fluid in transcritical conditions. The analysis is based on 84 experimental tests which cover a wide range of operating conditions of the plant: three evaporating levels (0, ?10, ?17 °C) at four compressor speeds (1150, 1300, 1450, 1600 rpm) over a range of discharge pressures from 74.2 to 104.9 bar.The paper presents the empirical model of the compressor and its validation with the experimental measurements, which allows the behaviour of the facility to be calculated with prediction errors below 5%. With the model, the effect of the SH_SC on the energy efficiency of the plant is evaluated by comparing the actual performance of the facility with the simulated behaviour of the plant without considering the SH_SC produced by the semihermetic compressor. The maximum estimated reductions, due to the SH_SC of the compressor, are a 20% in cooling capacity and a 23% in COP.     

Heat transfer and pressure drop during HFC refrigerant saturated vapour condensation inside a brazed plate heat exchanger

This paper presents the heat transfer coefficients and the pressure drop measured during HFC refrigerants 236fa, 134a and 410A saturated vapour condensation inside a brazed plate heat exchanger: the effects of saturation temperature (pressure), refrigerant mass flux and fluid properties are investigated. The heat transfer coefficients show weak sensitivity to saturation temperature (pressure) and great sensitivity to refrigerant mass flux and fluid properties. A transition point between gravity controlled and forced convection condensation has been found for a refrigerant mass flux around 20 kg/m²s that corresponds to an equivalent Reynolds number around 1600–1700. At low refrigerant mass flux (G_r < 20 kg/m²s) the heat transfer coefficients are not dependent on mass flux and are well predicted by the Nusselt [20] analysis for vertical surface: the condensation process is gravity controlled. For higher refrigerant mass flux (G_r > 20 kg/m²s) the heat transfer coefficients depend on mass flux and are well predicted by Akers et al. [21] equation: forced convection condensation occurs. In the forced convection condensation region the heat transfer coefficients show a 25–30% increase for a doubling of the refrigerant mass flux.The frictional pressure drop shows a linear dependence on the kinetic energy per unit volume of the refrigerant flow and therefore a quadratic dependence on mass flux.HFC-410A shows heat transfer coefficients similar to HFC-134a and 10% higher than HFC-236fa together with frictional pressure drops 40-50% lower than HFC-134a and 50–60% lower than HFC-236fa.     

Effects of operating parameters on the performance of a CO2 air conditioning system for vehicles

This paper deals with the effects of the operating parameters on the cooling performance that can be applied for a transcritical CO₂ automotive air conditioning system. The experimental conditions of the performance tests for a CO₂ system and components such as a gas cooler and an evaporator were suggested to compare with the performance of each at the standardized test conditions. This research presents experimental results for the performance characteristics of a CO₂ automotive air conditioning system with various operating conditions such as different gas cooler inlet pressures, compressor speeds and frontal air temperatures/flow rates passing through the evaporator and the gas cooler. Experimental results show that the cooling capacity was more than 4.9 kW and coefficient of performance (COP) was more than 2.4, at each optimum pressure of gas cooler inlet during idling condition. Also, the cooling capacity was about 7.5 kW and COP was about 1.7 at the optimum pressure of gas cooler inlet during driving condition when air inlet temperatures of gas cooler and evaporator were 45 °C and 35 °C, respectively. Therefore, we concluded that the automotive air conditioning system using CO₂ refrigerant has good performance. This paper also deals with the development of optimum high pressure control algorithm for the transcritical CO₂ cycle to achieve the maximum COP.     

Biodiesel from palm oil via loading KF/Ca–Al hydrotalcite catalyst

The solid base catalyst KF/Ca–Al hydrotalcite was obtained from Ca–Al layered double hydroxides and successfully used in the transesterification of methanol with palm oil to produce biodiesel. With the load of KF, the activity of Ca–Al mixed-oxides had been improved much. For the mass ratio 80 wt.%(KF·6H₂O to Ca–Al mixed-oxides) catalyst, under the optimal condition: 338 K, catalyst amount 5%(wt./wt. oil) and methanol/oil molar ratio 12:1, after 5 h reaction, the fatty acid methyl esters yield could reach 97.98%; for the mass ratio 100 wt.%(KF·6H₂O to Ca–Al mixed-oxides) ones, under the same reaction condition, only needed 3 h to get the FAME yield of 99.74%, and even only reacted 1 h, the FAME yield could obtain 97.14%.     

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.     

A novel miniaturized loop heat pipe

The research on a novel miniaturized loop heat pipe (LHP) consisted of an evaporator, a condenser, vapor and liquid lines is presented in this paper. In the LHP, the evaporator was separated into two parts of boiling and suction chambers by a vapor separator, which drove vapor to one-way flow to vapor line. Moreover, the bottom of evaporator was connected as the cycle channel of refrigerant. Thin copper plates with micro-fins as enhanced structures fabricated by the ploughing–extrusion (P–E) method were embedded in the boiling chamber. Accordingly, the copper fiber sintered felt fabricated by the solid-phase sintering of copper fibers with rough surface, was filled in the suction chamber of evaporator as the wick to provide the capillary force. In addition, the integral rhombic-shaped pillars fabricated by the milling, behaved as intensified condensation structures in the condenser. The startup and operation characteristics of LHP were tested under different heat loads and refrigerants. The experimental results showed that the highest temperature of evaporator reached 93.2 °C under the maximum heat load of 150 W.     

Development of 10-kWe preferential oxidation system for fuel cell vehicles

A preferential oxidation (PROX) reactor for a 10-kWe polymer electrolyte membrane fuel cell (PEMFC) system is developed. Pt-Ru/Al₂O₃ catalyst powder, with a size of 300–600 μm is applied for the PROX reaction. To minimize pressure drop and to avoid hot spots in the catalyst bed, the reactor is designed as a dual-staged, multi-tube system. The performance of the 10-kWe PROX unit is evaluated by feeding simulated gasoline reformate which contains 1.2 wt.% carbon monoxide (CO). The CO concentration of the treated reformate is lower than 20 ppm in the steady-state and is under 30 ppm at 65% load change. Hydrogen loss in the steady-state is about 1.5% and the pressure drop across the reactor is 4 psi. Start-up characteristics of the 10-kWe PROX system are also investigated. It takes 3 min to reduce the CO concentration to below 20 ppm. Several controllable factors are found to shorten the start-up time.     

Effect of partial bypass on the heat transfer performance of dehumidifying coils

This study extends a so called “partial bypass” concept to dehumidifying fin-and-tube heat exchangers. Tests are performed for a 2-row and 4-row fin-and-tube heat exchangers having plain fin configuration. The inlet dry bulb temperature is fixed at 25 °C while the inlet relative humidities are 50% and 80%, respectively. Test results showed that for a 2-row coil under RH = 50%, the corresponding heat transfer augmentation ratio (Q_R) and pressure penalty ratio (P_R) decrease with the rise of bypass ratio (BR). At a smaller frontal velocity, the regime of appropriate bypass ratio where Q_R > 1 and P_R < 1 is more apparent. The effect of partial bypass decreases as the velocity increases. As the velocity gets close to 2 m s^− 1, it shows that the performance at high bypass ratio becomes more efficient. At the same time, when bypass ratio becomes lager, the tendency of the decreasing of ΔP is more drastic than the decreasing of Q. Nevertheless, not for all the cases that the performances get better as the bypass ratio increases. For V_fr = 4 m s^− 1, the experiment results show that the overall effect is less efficient than at V_fr = 2 m s^− 1. The major reason is caused by the flow pattern. The rise of bypass airflow is quite large which may act as an air curtain to distort the main airflow, and result in higher pressure drop. This implies that the design of the bypass design is quite imperative in the practice of real applications. For a 4-row heat exchanger, the experiment data of Q_R and P_R for the 4-row coil performs better than a 2-row coil.     

Pool boiling of R-123/oil mixtures on enhanced tubes having different pore sizes

The effect of enhanced geometry (pore diameter, gap width) is investigated on the pool boiling of R-123/oil mixture for the enhanced tubes having pores with connecting gaps. Tubes having different pore diameters (and corresponding gap widths) are specially made. Significant heat transfer degradation by oil is observed for the present enhanced tubes. At 5% oil concentration, the degradation is 26–49% for T_sat = 4.4 °C. The degradation increases 50–67% for T_sat = 26.7 °C. The heat transfer degradation is significant even with small amount of oil (20–38% degradation at 1% oil concentration for T_sat = 4.4 °C), probably due to the accumulation of oil in sub-tunnels. The pore size (or gap width) has a significant effect on the heat transfer degradation. The maximum degradation is observed for d_p = 0.20 mm tube at T_sat = 4.4 °C, and d_p = 0.23 mm tube at T_sat = 26.7 °C. The minimum degradation is observed for d_p = 0.27 mm tube for both saturation temperatures. It appears that the oil removal is facilitated for the larger pore diameter (along with larger gap) tube. The highest heat transfer coefficient with oil is obtained for d_p = 0.23 mm tube, which yielded the highest heat transfer coefficient for pure R-123. The optimum tube significantly (more than 3 times) outperforms the smooth tube even with oil. The heat transfer degradation increases as the heat flux decreases.     

Influence of carbon nanotubes on nucleate pool boiling heat transfer characteristics of refrigerant–oil mixture

Influence of carbon nanotubes (CNTs) on nucleate pool boiling heat transfer characteristics of refrigerant–oil mixture was investigated experimentally. Four types of CNTs with the outside diameters from 15 nm to 80 nm and the lengths from 1.5 μm to 10 μm were used in the experiments. Test conditions include CNTs mass fractions in the CNTs nanolubricant from 0 to 30 wt% and CNTs nanolubricant mass fractions from 0 to 5 wt%. The experimental results indicate that the presence of CNTs enhances the nucleate pool boiling heat transfer coefficient of R113-oil mixture by a maximum of 61% under the present test conditions, and the enhancement increases with the decrease of CNTs outside diameter or the increase of CNTs length. For fixed CNTs physical dimension, the enhancement increases with the increase of CNTs mass fraction in the CNTs nanolubricant or the decrease of CNTs nanolubricant mass fraction. A correlation for predicting the nucleate pool boiling heat transfer coefficient of refrigerant–oil mixture with CNTs is proposed, and it agrees with 96% of the experimental data within a deviation of ±10%.     

Design and performance of a solar-powered heating and cooling system using silica gel/water adsorption chiller

In this paper, a solar-powered compound system for heating and cooling was designed and constructed in a golf course in Taiwan. An integrated, two-bed, closed-type adsorption chiller was developed in the Industrial Technology Research Institute in Taiwan. Plate fin and tube heat exchangers were adopted as an adsorber and evaporator/condenser. Some test runs have been conducted in the laboratory. Under the test conditions of 80 °C hot water, 30 °C cooling water, and 14 °C chilled water inlet temperatures, a cooling power of 9 kW and a COP (coefficient of performance for cooling) of 0.37 can be achieved. It has provided a SCP (specific cooling power) of about 72 W/(kg adsorbent). Some field tests have been performed from July to October 2006 for providing air-conditioning and hot water. The efficiency of the collector field lies in 18.5–32.4%, with an average value of 27.3%. The daily average COP of the adsorption chiller lies in 33.8–49.7%, with an average COP of 40.3% and an average cooling power of 7.79 kW. A typical daily operation shows that the efficiency of the solar heating system, the adsorption cooling and the entirely solar cooling system is 28.4%, 45.2%, and 12.8%, respectively.     

Effects of ethylene glycol ethers on diesel fuel properties and emissions in a diesel engine

The effect of ethylene glycol ethers on both the diesel fuel characteristics and the exhaust emissions (CO, NO_x, smoke and hydrocarbons) from a diesel engine was studied. The ethers used were monoethylene glycol ethyl ether (EGEE), monoethylene glycol butyl ether (EGBE), diethylene glycol ethyl ether (DEGEE). The above effect was studied in two forms: first by determining the modification of base diesel fuel properties by using blends with oxygen concentration around 4 wt.%, and second by determining the emission reductions for blends with low oxygen content (1 wt.%) and with 2.5 wt.% of oxygen content. The addition of DEGEE enhances base diesel fuel cetane number, but EGEE and EGBE decrease it. For concentrations of ?4 wt.% of oxygen, EGEE and diesel fuel can show immiscibility problems at low temperatures (?0 °C). Also, every oxygenated compound, according to its boiling point, modifies the distillation curve at low temperatures and the distillate percentage increases. These compounds have a positive effect on diesel fuel lubricity, and slightly decrease its viscosity. Blends with 1 and 2.5 wt.% oxygen concentrations were used in order to determine their influence on emissions at both full and medium loads and different engine speeds. Generally, all compounds help to reduce CO, and hydrocarbon emissions, but not smoke. The best results were obtained for blends with 2.5 wt.% of oxygen. At this concentration, the additive efficiency in decreasing order was EGEE > DEGEE > EGBE for CO emissions and DGEE > EGEE > EGBE for hydrocarbon emissions. For NO_x, both its behaviour and the sequence are opposite to that of CO.     

Convective gas cooling heat transfer and pressure drop characteristics of supercritical CO2/oil mixture in a minichannel tube

The effects of PAG oil concentration on the convective gas cooling heat transfer and the pressure drop characteristics of supercritical CO₂/oil mixture in minichannel tube were investigated. The test results showed that the average gas cooling heat transfer coefficient was decreased by 20.4% and the average pressure drop was increased by 4.8 times when the oil concentration was increased from 0 to 4 wt.%. The effects of the oil concentration on the convective gas cooling heat transfers and the pressure drops of the supercritical CO₂/oil mixture in minichannel tubes were experimentally confirmed to be significant.