Experimental investigation on heating performance of gas-injected scroll compressor using R32, R1234yf and their 20wt%/80wt% mixture under low ambient temperature

In this paper, an integrated gas-injected scroll compressor heat pump system using R1234yf, R32 and its binary mixtures as working fluid was developed and their heating performances under low ambient temperature were quantitatively evaluated. A composite test system consisting of second-refrigerant calorimeter and water-cooled condenser was used to test the system working performance. The condensing temperature, evaporating temperature, compressor power input and other variables were analyzed to evaluate the system heating capability and energy efficiency. Test results showed that the R1234yf system can run at an evaporating temperature of −25 °C. R1234yf/R32 mixture can run at an evaporating temperature of −20 °C and it has the highest heating COP value among other refrigerants; R1234yf/R32 gas injection system provided very significant performance improvements for heating performance, compared with no gas injection, the heating capacity and heating COP can improve 16%~20% and 13%~16%, respectively.     

Theoretical study on a frost-free household refrigerator-freezer

This study deals with a frost-free household refrigerator-freezer, in which frosting can be retarded by dehumidifying the air before it enters the evaporator of the refrigerator-freezer via a desiccant-coated heat exchanger. Because the desiccant can be regenerated via the condensation heat of the refrigerant (which is exhausted into ambient air in conventional household refrigerator-freezers), the proposed system can achieve high energy efficiency. Calculations show that the coefficient of performance of this system (COP) is within the range 1.5–2.5 at an ambient temperature of 15–35 °C. Moreover, it is found that the relative humidity of the refrigerator air (RH_RA) and the temperature of freezer air (T_FA) have a significant effect on COP: COP decreases by about 13% when RH_RA varies from 0.4 to 0.8 and by 10% when T_FA runs from −18 to −23 °C.     

Experimental investigation of solar driven desiccant air conditioning system based on silica gel coated heat exchanger

A silica gel coated heat exchanger based air conditioning system driven by the evacuated tube solar water heater has been experimentally investigated. The system has been operated for two different modes namely cooling with dehumidification mode and heating with humidification mode in summer and winter season respectively. The system performance is analyzed in terms of regeneration rate, dehumidification rate and thermal coefficient of performance (COP_th). Experimental results demonstrated that, for cooling and dehumidification mode, the process air is cooled by an average temperature of 8.5 °C. A better dehumidification rate can be achieved by using pre-cooling before dehumidification process. Post-cooling after dehumidification process is found to be advantageous for cooling capacity and COP_th. For heating with humidification mode, the process air is heated by an average temperature of 13.3 °C with an average increment in humidity ratio of 1.9 g/kg. It is found that the average COP_th of the system is 0.45 and 0.87 for cooling and heating mode respectively.     

Experimental study on a two-stage rotary desiccant cooling system

The objectives of this study were to evaluate the performance of a novel two-stage rotary desiccant cooling (TSRDC) system and to obtain useful data and experiences for practical application. Newly developed compound desiccant (silica gel–haloids) was adopted in the system. An experimental set-up was built and used to test the system performance under three typical environmental conditions. System performances were evaluated in terms of moisture removal D and thermal coefficient of performance COP_th. It has found that the required regeneration temperature of TSRDC system is low and COP_th of the system is high. Regeneration temperatures from 65 °C to 80 °C, 65 °C to 75 °C and 80 °C to 90 °C were recommended for each environmental condition. In addition, the effects of some important operating parameters, such as inlet temperature and humidity ratio of process and regeneration air, on system performance were also investigated in this study.     

Cellulose-pad water cooling system with cold storage

The present study develops a cooling system using water as the working medium which is cooled at night by cellulose-pad cooling tower (CWCT) and stored for cooling application at daytime. That is, it utilizes the natural energy drawn from diurnal ambient air temperature difference. A cooling system was built and tested. It is found that the coefficient of performance of CWCT for heat dissipation of water at night, COP_nt, is between 3.8 and 11 and varies linearly with the evaporation temperature glide D_G (difference between cold water temperature in the storage tank and wet-bulb temperature of ambient air). The COP for room cooling at daytime run with air cooler in a room, COP_day, is between 8.8 and 12.6. For day cycle operation, the measured overall cooling COP_o is 5.1. COP_o is expected to reach 9.4 at room temperature 45 °C.     

Theoretical analyses of a new two-stage absorption-transcritical hybrid refrigeration system

In this context, a two-stage absorption-transcritical hybrid refrigeration system is proposed. R744 is chosen as a refrigerant for the transcritical heat pump subsystem and LiBr-H₂O working pair for the two-stage absorption refrigeration subsystem. Based on the mathematical and physical models, theoretical investigation is carried out on its performance. The main effects are discussed on COP_net (the ratio of cooling capacity powered by low-grade heat to the low-grade heat consumption for the hybrid system) and COP_mt (the ratio of cooling capacity powered by mechanical work to the mechanical work consumption for the hybrid system). Comparing with the normal two-stage absorption refrigeration system, theoretical results show that COP_net could be improved up to about 55% when the refrigeration temperature is 7 °C. In addition, COP_mt are more than 50% higher than that of the conventional transcritical refrigeration system. It is also found that both 45–55 °C low-grade heat and condensing heat could be used as actuating heat of the two-stage absorption refrigeration subsystem.     

Theoretical study of R32 in an oil flooded compression cycle with a scroll machine

R32 is regarded as a potential alternative for R410A, but it has a low slope of isentropic line, high superheat inside a compressor and thus a high discharge temperature. These disadvantages limit its wider adoption. In order to improve the performance of R32 air conditioner, oil flooded compression with regenerator has been suggested. A single stage oil flooded compressor model is developed to obtain a more accurate system-level improvement. In the compressor model, the heat transfer losses between shell and ambient, suction gas and motor, and high-pressure and low-pressure cylinders are considered. By means of parametric studies, it was found that the novel cycle resulted to be beneficial to increase the compressor internal superheating, to decrease the compressor heat losses and to improve its overall isentropic efficiency while cooling capacity or heating capacity is degraded. COP_h improvement can reach up to 16.4% for an evaporating and condensing temperatures of −25 °C and 45 °C, respectively. The discharge temperature resulted to be lower than 110 °C. In addition, a thorough comparison between R32 and R410A with both novel and baseline systems has been carried out. The results indicate that the novel cycle has potential benefits for applications in R32 air conditioners.     

Experimental and numerical analysis of an air-cooled double-lift NH3–H2O absorption refrigeration system

The prototype of an air-cooled double-lift NH₃–H₂O absorption chiller driven by hot water at low temperature is presented. The main objective of the study is to illustrate the experimental performances of the prototype under different operating conditions. A mathematical model of the cycle is developed, along with a procedure for the identification of otherwise difficult to measure data, with the purpose of providing the complete picture of the internal thermodynamic cycle. The combined experimental and numerical data allowed assessing the effects on the thermodynamic cycle with varying operating conditions. The unit operated steadily with chilled water inlet 12 °C, outlet 7 °C, air temperature between 22 °C and 38 °C, and hot water driving temperatures between 80 °C and 90 °C. The reference cooling capacity at air temperature of 30 °C is 2.5 kW, with thermal COP about 0.3 and electrical COP about 10.     

Electrical properties of TeAsGe films

The effect of temperature T, electrode separation d and substrate temperature T_s on the I–V characteristics of amorphous films 3500 Å thick, vapour deposited from the alloy TeAsGe (53:36:11 at. %) onto a glass substrate at room temperature, were investigated. The material displayed the behaviour of a negative resistance device with a memory. The behaviour for T=constant is described by the relation V = CIexp (?αI), where C and α are constants for a specimen at constant temperature. The threshold voltage at which the off state transforms to the negative resistance state decreased with T according to the relation V_th = V₀exp (E_v/2 kT), where E_v = 0.21 eV. V_th increased with d and decreased with T_s and was related to changes in resistance and structure. Microscope examination showed the formation of filaments containing recrystalized structure arising from Joule heating.     

Effect of salt concentration and exposure temperature on adhesion and cytotoxicity of positively charged nanoparticles toward yeast cells

We have comprehensively scrutinized the effect of NaCl salt concentration (C_NaCl = 5–600 mM) and exposure temperature (T_exp = 4 and 25 °C) on the adhesion and cytotoxicity of positively charged nanoparticles (NPs) of 100-nm diameter toward two yeast strains (Saccharomyces cerevisiae and Schizosaccharomyces pombe) during exposure of t_exp = 0.5–24 h. On the short-time exposure toward S. cerevisiae at T_exp = 25 °C, the number of the NPs adhered to a yeast cell (N_adh) monotonically decreased with C_NaCl to become negligibly small at C_NaCl ≥ 150 mM. The greater value of N_adh resulted in the higher cell mortality. Similar results were obtained for S. pombe at C_NaCl = 5–100 mM. Interestingly, N_adh for S. pombe increased with C_NaCl (=100 to 600 mM), where the cells considerably covered with the NPs showed relatively low mortalities during the short-time exposure. The favorable NaCl concentration was found as C*_NaCl = 150 and 100 mM for S. cerevisiae and S. pombe, respectively, to survive in the aqueous suspensions of positively charged NPs during 24 h. The lower-temperature exposure (T_exp = 4 °C) for each exposure duration caused the significantly decreasing mortality of every yeast strain at the NaCl concentrations around C*_NaCl.     

Feasibility study and performance evaluation of low capacity water–LiBr absorption cooling systems functioning in different Algerian climate zones

A performance analysis was carried out on water–LiBr absorption chillers performing in the five different climate zones in Algeria. A 17.6 kW single-effect and a 16 kW double-effect commercial absorption chillers were simulated. In climate zones E1 and E2, the single-effect and double-effect chillers supplied 37% and 91%, respectively, of their nominal capacity to produce chilled water at 7 °C. In the hot climate zones E3, E4 and E5, it was not feasible for either of the chillers to produce chilled water at 7 °C. By increasing the chilled water temperature to 12 °C both absorption chillers were able to operate in climate zones E3 and E4. The single-effect chiller reached 45% of its nominal capacity in zone E3 and 33% in zone E4. The double-effect chiller delivered 80% of its nominal capacity in both climate zones. Neither of the chillers was able to operate under the thermal conditions of climate zone E5.     

Experimental evaluation of a variable effect LiBr–water absorption chiller designed for high-efficient solar cooling system

A variable effect LiBr–water absorption chiller is studied in this paper based on a real developed 50 kW prototype. The chiller is designed specifically for the high-efficient utilization of the solar power with variable temperature. It can obtain the optimized COP and cooling power under different heat source temperatures. The construction, circulation and testing system of the chiller were introduced. A typical running condition of the chiller from the starting to the steady operating was given to show the dynamic performance. Several groups of the temperatures and COPs were given to show the steady state performance. These data showed that the COP increased from 0.69 to 1.08 under generation temperature from 95 °C to 120 °C. Besides, the effects of chilled water temperature, cooling water temperature, pump frequency and opening of valve on COP and cooling power were analyzed respectively.     

Generation of thermal strains in carbon fibre-reinforced bismaleimide (PMR-15) composites: Part 1: The determination of residual thermal strains in cross-ply laminates

The level of residual thermal strain which develops in PMR-15 cross-ply laminates has been determined from the curvature of unbalanced 0°/90° beams. The hysteresis in the temperature dependence of the beam deflection has been attributed to the plasticizing effect of entrapped curing volatiles (mainly water and methanol). Vacuum drying has been shown to remove the hysteresis but increase the magnitude of the residual thermal strain. The latter appears to result from a complex interaction between the enhancement in matrix expansion coefficient, a reduction in transverse modulus and the strain-free temperature (T₁). Since T₁ is in the region of 300°C it has not proved possible to demonstrate its reduction directly. These values of thermal strain have been compared with predictions obtained from measured transverse and matrix expansion coefficients (α_t, α_m). The presence of residual volatiles also appears to be responsible for some variability in the temperature dependence of the matrix-dominated expansion coefficients. Post-curing at higher temperatures gives rise to better reproducibility in the values of α_m, α_t and ϵ_{t 1}^th. The role of these volatiles is discussed further in Parts 2 and 3.     

Condensation in two phase and desuperheating zone for R1234ze(E), R134a and R32 in horizontal smooth tubes

Condensation is usually assumed to begin when the bulk enthalpy reaches the saturated vapor enthalpy, which leads to discontinuity of heat transfer coefficient calculation in modeling. This paper addresses the discontinuity by showing the presence of condensation in desuperheating region when the wall temperature decreases below the saturation temperature at any operating condition. The experiments have been conducted with R134a, R1234ze(E) and R32 for mass fluxes of 100–300 kgm⁻² s⁻¹, saturation temperatures of 30^°C–50 °C and from x = 0.05 to superheat of 50 °C in a horizontal smooth tube with 6.1 mm inner diameter. R134a is observed to have approximately 10% higher and 20% lower HTC compared to R1234ze(E) and R32 respectively. Cavallini correlation predicted the data within an accuracy of 12% while Kondo-Hrnjak correlation predicted HTC for condensation in de-superheating zone within accuracy of 23%.     

Solidification pathway and phase transformation behavior in a beta-solidified gamma-TiAl based alloy

The phase transformation behavior of an as-cast Ti-42Al-5 Mn(at.%) alloy after subsequent quenching from 1380 ℃ to 1000 ℃ was investigated based on the differential thermal analysis(DTA),electron probe micro analyzer-backscattered electrons(EPMA-BSE),transmission electron microscope(TEM) and X-ray diffraction(XRD).The results show that,the solidification path can be summarized as follows:Liquid→Liquid+β→β→β→α→β+α+γ→β_o+α_2+γ→β_o+γ+α_2/γ→β_o+γ+α_2/γ+β_(o,sec),with the phase transformation α→β temperature(T_β)=1311℃,phase transformation γ→β temperature of(T_(γsolv))=1231℃,phase transformation α_2→α or β_o→β temperature(T_(α2→α)/T_(β_o→β))=1168 C,eutectoid temperature(T_(eut))=1132 ℃ and T_(α_2/γ→βo,sec)≈1 120℃.In comparison with Ti-42 Al alloy,the T_(eut) and T_(γsolv)are slightly increased while both the Tp is decreased obviously by 5% Mn addition.When quenched from the temperature of 1380-1260 ℃,the martensitic transformation β→α' could occur to form the needlelike martensite structure in β area.This kind of martensitic structure is much obvious with the increase of temperature from 1260℃ to 1380 ℃.When the temperature is below T_(γsolv)(1231℃),the γ grains would nucleate directly from the β phase.For the temperature slightly lower than T_(eut)(1132℃),the dotted β_(o,sec) phases could nucleate in the lamellar colonies besides the γ lamellae precipitated withinα_2 phase.Finally,at room-temperature(RT),the alloy exhibits(p_o+α_2+γ) triple phase with microstructure of β_o+lamellae+γ,of which the lamellar structure consists of α_2,γ and β_(o,sec) phases.The phase transformation mechanisms in this alloy,involving β→α',β→γ,α_2→α_2/γ and α_2→β_(o,sec) were discussed.     

Optimal design of an absorption chilled water storage air conditioning system driven by waste heat

In order to meet both economic and energy requirements, this study has proposed an optimal design to minimize the sum of the initial and operation energy costs for a 1200 refrigeration ton chiller and 12 water storage tanks in an absorption chilled water storage air conditioning system. Various power consumption calculation methods for the main devices are included to predict the performance of this equipment under different operating conditions. In addition, the performance curves of the water storage tank under the storage and discharge modes are calculated using Fluent software. The article uses five control strategies for a cooling tower along with three hot water inlet temperatures of a generator to simulate the optimal design of a system. The results show that the least power (10,336 kWh) is consumed when the cooling tower’s outlet temperature is 32°C and the generator’s inlet hot water temperature is 105°C.     

Experimental evaluation of a triple-state sorption chiller

This paper presents the experimental testing results of a novel triple-state sorption chiller with integrated cold storage. The performance of the chiller was measured for hot water inlet temperatures between 65 °C and 95 °C, heat rejection inlet temperatures between 15 °C and 35 °C, and chilled water inlet temperatures between 10 °C and 25 °C. An empirical model was developed for implementation in the TRaNsient SYstem Simulation (TRNSYS) software. To validate the model, a five-hour experimental charge test was conducted during which the hot water and heat rejection inlet temperatures were continuously varied. The model was able to predict the total heat input and heat rejection energy to within 0.7% and 1.3% of the experimentally measured values, respectively.     

La0.7Sr0.3MnO3 Nanoparticles Synthesized via the (Pechini) Polymeric Precursor Method

In this paper, we report the preparation of La_0.7Sr_0.3MnO₃ colossal magnetoresistance nanoparticles by means of the polymeric precursor method, at a temperature of 650 °C. Rietveld refinement of the X-ray powder diffraction spectra shows that the chemically-synthesized manganite is single-phase with the space group R3C. By using the peak broadening technique and Scherrer’s formula, a grain size of ～15 nm was estimated. The ferromagnetic to paramagnetic transition is sharp with a Curie temperature of T _C ～360 K. In spite of the low annealing temperature, the insulator-metal-like transition temperature (T _p ) is close to T _C . Transport measurements on the prepared samples show a magnetoresistance change of ～10 % at room temperature in a field of 2 T. This high-field MR is probably due to grain-boundary effects.     

The epitaxial growth of germanium and silicon on an Ag(111) film on a mica substrate

With the aim of finding a method of obtaining self-supporting single-crystal films of silicon for solar cells we studied the epitaxial growth of silicon and germanium prepared by evaporation in ultrahigh vacuum onto an Ag(111) film evaporated in situ onto a mica substrate cleaved in air. The films were examined mainly by reflection high energy electron diffraction. Silicon and germanium films 50–200 Å thick were composed of crystallites with two main orientations relative to the substrate and unoriented crystallites in varying proportions depending on the substrate temperature T_s and the previous heat treatment temperature T_H of the mica. Nearly single-crystal films of silicon could be obtained for T_s = 350 °C and T_H = 250 °C. The sticking coefficient for silicon on silver was found to decrease almost to zero for T_s = 420 °C with T_H = 250 °C. No single-crystal films of germanium were obtained.     

Solubility characteristics of R22-DMF refrigerant-absorbent combinationCaractéristiques de solubilité du mélange frigorigéne-absorbant R22-diméthylformamide

This Paper presents the solubility data of Refrigerant 22 (R22) - dimethyl formamide (DMF) measured over a wide range of temperature (?25°C to 120°C) and over a complete range of composition. The mixtures show large negative deviation from Raoult's law and there is large difference in normal boiling point its components. Thus this combination is suited for vapour absorption refrigerant systems. The In P-1/T diagram and temperature-composition diagram for the mixtures of R22-DMF have also been plotted for use in the analysis of vapour absorption systems.