The performance of vapor compression refrigeration system with controllable speed compressor and vapor and liquid refrigerant injection techniques is experimentally investigated. For this purpose, a 5-tons (17.6 kW) split air conditioner unit charged with R-22 refrigerant was modified and equipped with, frequency inverter, secondary capillary tube, and liquid pressure amplification pump (LPA). Vapor refrigerant was injected into the accumulator with three injection mass ratios 2, 3, and 4%. LPA pump was used to inject liquid refrigerant from the condenser outlet to the discharge line with injection mass ratios 0.5%, 1.5%, and 2%. The compressor speed was controlled by frequency inverter, where the range of the frequency used was from 35 to 60 Hz with 5 Hz step. The results showed that the coefficient of performance (COP) of the modified system using vapor injection technique was improved by 11.26% compared to the conventional system, and the suction temperature reduced from 10.3 to -0.1°C with 3% vapor injection ratio. The best enhancement in COP was 9.9% for the 0.5% liquid injection ratio. Reduction of the compressor speed leads to improve the COP by 18% and reduces the compressor power by 36.4% at frequency 35Hz. Using the vapor injection with variable speed compressor improved the COP of the modified system by 75% at 35 Hz and 2% injection mass ratio.
An experimental investigation of refrigerant R-134a two-phase flow condensation heat transfer coefficient and pressure drop in condenser tube section of refrigeration system under different operating conditions is presented. The experimental and theoretical investigations are based on test conditions in range of 10 -17 kW/m 2 for heat flux, 42-63 kg/m 2 s for mass flux, vapor quality 1-0.03 and saturation temperature 44 to 49˚C. The experimental tests are conducted on test rig supplied with a test section to simulate the water cooled double pipe heat exchanger, which is designed and constructed in the present work. "The experimental results have revealed that, the heat flux and mass flux have significant impacts on the heat transfer coefficient. "The heat transfer coefficient was increased with increase in heat flux and mass flux at prescribed test conditions, where the enhancement in heat transfer coefficient was about 47% and 14% for relatively higher heat flux and mass flux, respectively. "The enhancement in the heat transfer coefficient was about 51% for relatively lower saturation temperature 45.97˚C and 43% for higher vapor quality 0.88 compared to other values at constant test conditions. "The pressure drop was higher in the range of 12% and 49% for relatively higher mass flux and heat flux respectively. "The present work results have validated by comparison with predictive models and with similar research work results and the comparison has revealed an acceptable agreement.
The impact of the solar photovoltaic cell temperature on module output performance parameters is investigated experimentally under conditions of Baghdad city climate in the present study. The tests are conducted using a test rig for Polycrystalline silicon solar panel with rated capacity 300W and equipped with measuring devices. Test conditions are based on three months duration, June, July and August of year 2019 through the period from 8AM to 4PM per day. This period represents the highest ambient temperatures through summer months in Baghdad city in Iraq. The results have shown that, under the test conditions for cell temperature with range 45-65°C a significant reduction in the output power, efficiency and fill factor with module temperature rise were observed. The loss in the open circuit voltage and output power of the photovoltaic module was about-0.104/°C and-1.3/°C respectively. The reductions in the current and voltage at maximum output power with temperature increasing were about-5.24% and-5.45% respectively. The drops in the efficiency and fill factor of the photovoltaic module due to the module temperature increase were 9.62% and 12.96% respectively compared to that at standard conditions. The temperature coefficient of the maximum power was-0.52% for the solar photovoltaic module under test conditions.
The heat transfer performance of heat exchanger is greatly depending on the thermal conductivity and heat transfer capacity of the working fluid. One of the important methods to improve the thermal conductivity of the heat transfer fluid is by adding nano particles of materials with high thermal conductivity. In the present study, an experimental work was conducted to investigate the performance enhancement of double pipe heat exchanger using Alumina (Al2O3) and Copper oxide (CuO) nano particles mixed with engine oil as a working fluid. The thermal performance of heat exchanger was investigated at particles concentrations 0.05% and 0.1% of Al2O3 and CuO nano particles to determine the most effective factors on heat transfer enhancement. The results revealed that, the enhancement percentage in Nusselt number (Nu) for nanofluid at 0.05% particles concentration compared to pure oil was 9% and 6% for CuO and Al2O3 nanofluids respectively. While at 0.1% concentration, the enhancement percentage in Nu was about 17% and 15% for CuO and Al2O3 nanofluids respectively. The enhancement percentage in value of overall heat transfer coefficient for CuO nanofluid was 6.5% compared to Al2O3 nanofluid at 0.1% concentration. The enhancement percentage in the heat exchanger effectiveness was about 12% for CuO nanofluid compared with that for Al2O3 at 0.1% concentration. Adding the CuO and Al2O3 nano particles has improved the thermal conductivity of base fluid (oil) and led to a significant enhancement in heat transfer rate and thermal performance of the heat exchanger.
There are many approaches by which the heat exchanger can be modeled depending on how much information is available to start with. Grey-box model represents one of these approaches which is considered in the present study to modeling the unsteady operation heat exchanger. The available measurements of an unidentified heat exchanger integrated with a process involving oil circulation were statistically analyzed to extract the information that could aid in its identification. The proposed heat exchanger system included a hot oil stream coming from the thermal unit process and cooled by a cold-water circuit. The objective of this study is to develop a grey box model for the unspecified heat exchanger with a suitable nonlinear state-space structure and solved numerically using MATLAB-19 software and engineering equation solver (EES). Suitable parameters included oil and water inventories, heat addition, and conductance divisor factor are chosen to fully identify the heat exchanger from measured temperatures and flow rates. The parameters used to identify the oil process included the oil mass inventory within the process and the quantity of heat added. While for the heat exchanger; oil, water, and solid thermal masses were determined along with a conductance divisor to close its model. The results revealed that, comparing the model output with the measured data was satisfactory. The effect of 10% increment in the oil process heat as external excitation on a heat exchanger can only be controlled by water and oil flow rates and any fluctuation in inlet water temperature is insignificant and considered as a noise disturbance. An increment of 19% in the oil flowrate with unchanged water flowrate resulted in an increase in oil outlet temperature by about 4%. applying a noise of about ±10% on inlet water temperature to the system resulted in insignificant effects on the oil temperatures, and therefore it could be considered as uniform temperature. An effective control system to manage the heat exchanger and therefore the process can be designed according to the predictions of changing the water and oil flow rate responses.
The heat transfer characteristics of R134a flow boiling in a horizontal tube of an evaporator section for a refrigeration system of 310-W capacity are investigated experimentally and numerically. The experimental work was conducted using an evaporator tube test section of inner diameter 5.8[Formula: see text]mm and length 600[Formula: see text]mm. The ranges of investigated experimental data for heat flux, mass flux, saturation temperature and vapor quality were 13.8–36.6[Formula: see text]kW/m2, 52–105[Formula: see text]kg/m2[Formula: see text][Formula: see text][Formula: see text]s, [Formula: see text]–[Formula: see text]C and 0.2–1, respectively. Numerical analysis was based on two-phase flow turbulent model and this model was solved using the Ansys-18 code. The results showed that the effects of heat flux, mass velocity and saturation temperature on local heat transfer coefficient and pressure drop were greater compared to that of the refrigerant vapor quality. The enhancements in local heat transfer coefficient due to the increase in heat flux, mass and saturation temperature were 38%, 57% and 64%, respectively, within the prescribed test conditions. The influence of mass flux variation on pressure drop along the evaporator channel was higher in the range of 27% compared to the heat flux effect. The average deviations between experimental and numerical results of heat transfer coefficient and pressure gradient were 14% and 17%, respectively, while the same between the experimental and predicted results were 16% and 33%, respectively.
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