Flow Regimes and Transitions for Two-Phase Flow of R152a During Condensation in a Circular Minichannel

Two-phase flow regimes were experimentally investigated during the entire condensation process of refrigerant R152a in a circular glass minichannel. The inner and outer diameters of the test minichannel were 0.75 and 1.50 mm. The channel was 500 mm long to allow observation of all the two-phase flow regimes during the condensation process. The experiments used saturation temperatures from 30 to 50°C, a mass flux of 150 kg/(m2·s) and vapor qualities from 0 to 1. The annular, intermittent and bubbly flow regimes were observed for the experimental conditions in the study. The absence of the stratified flow regime shows that the gravitational effect is no longer dominant in the minichannel for these conditions. Vapor-liquid interfacial waves, liquid bridge formation and vapor core breakage were observed in the minichannel. Quantitative measurements of flow regime transition locations were carried out in the present study. The experiments also showed the effects of the saturation temperature and the cooling water mass flow rate on flow regime transitions. The results show that the annular flow range decreases and the intermittent and bubbly flow ranges change little with increasing saturation temperature. The cooling water mass flow rate ranging from 38.3 kg/h to 113.8 kg/h had little effect on the flow regime transitions.

Enhancing the renewable energy payback period of a photovoltaic power generation system by water flow cooling

A photovoltaic system which enjoys water flow cooling to enhance the performance is considered, and the impact of water flow rate variation on energy payback period is investigated. The investigation is done by developing a mathematical model to describe the heat transfer and fluid flow. A poly crytalline PV module with the nomical capacity of 150 W that is located in city Tehran, Iran, is chosen as the case study. The results show that by incresing water flow rate, EPBP declines first linearly, from the inlet water flow rate of 0 to 0.015 kg.s-1, and then, EPBP approaches a constant value. When there is no water flow cooling, EPBP is 8.88, while by applying the water flow rate of 0.015 kg.s-1, EPBP reaches 6.26 years. However, only 0.28 further years decreament in EPBP is observed when the inlet water mass flow rate becomes 0.015 kg.s-1. Consequently, an optimum limit for the inlet water mass flow rate could be defined, which is the point the linear trend turns into approaching a constant value. For this case, as indicated, this value is 0.015 kg.s-1.

Recommendations for Running a Tandem of Adsorption Chillers Connected in Series and Powered by Low-Temperature Heat from District Heating Network

In this paper, we investigate implications of running a cooling system of two silicagel/water adsorption chillers powered by a district heating network. The devices are connected in series, i.e., the heating water output from the primary chiller is directed to the secondary one. In consequence, the secondary device must deal with an even lower driving temperature and with temperature fluctuations caused by the primary device. We have evaluated three factors that influence the operation of those coupled devices: synchronization of their operating cycles, selection of their cycle time allocations (CTAs), and changing the heating water mass flow rate. Numerical analyses indicate that the performance of the secondary chiller drops significantly if the coupled devices that use the same CTA run asynchronously. The decrease is largest if the shift between the operating cycles is x=0.375 and x=0.875. We found that it is possible to reduce the negative influence of the asynchronous operation by implementing different CTA in each chiller. The best performance is achieved if the primary chiller uses an adsorption time to desorption time ratio f=1.0 and the secondary chiller uses f = 0.6–0.7.

Bi-fluid cooling effect on electrical characteristics of flexible photovoltaic panel

A photovoltaic (PV) system integrated with a bi-fluid cooling mechanism, which is known as photovoltaic thermal (PVT) system, was investigated. The electrical characteristics of flexible solar panel were evaluated for PV and PV with bi-fluid (air and water) cooling system. The integration of monocrystalline flexible solar panel into both systems was tested under a fixed solar radiation of 800 W/m2. A total of 0.04–0.10 kg/s of air flow was utilised in PV with cooling system with a fixed water mass flow rate of 0.025 kg/s. The efficiencies of flexible panel for PV and PV with cooling system were explored. For PV with bi-fluid flow, the highest obtained efficiency of module was 15.95% when 0.08 kg/s of air and 0.025 kg/s of water were allowed to flow through the cooling system. Compared with PV without cooling mechanism, the highest efficiency of module was 13.35% under same solar radiation. Current–voltage and power graphs were also plotted to present the electrical characteristics (current, voltage and power) generated by both systems.

Solar Air-Heated Humidification-Dehumidification Desalination Unit

A solar powered desalination unit which working on a humidification –dehumidification technique (HDH) is one of the most important techniques used in brackish and seawater desalination especially in remote and rural areas. In the present study, a test-rig was designed and constructed for conducting a set of experiments on a solar assisted desalination unit working on a HDH principle under the prevailing conditions of Tajoura-Libya. Experiments were carried out on specified days in March, 2019 at the laboratories of Center for Solar Energy Research and Studies (CSERS) at Tajoura. The effect of different design parameters and operating conditions on the performance of the unit and its productivity is closely investigated and interpreted. Results show that the productivity of the HDH unit decreases by increasing the process air mass flow rate. A significant improvement in the productivity of the unit is noticed when the feed water mass flow rate to the humidifier is increased. Furthermore, initial water temperature inside the tank has a remarked effect on the productivity of the unit. In order to obtain a reasonable amount of fresh water, the temperature of the water inside the tank should be increased. Increasing the cooling water mass flow rate to the dehumidifier leads to a corresponding decrease in the surface temperature of the cooling coil and hence the productivity of the unit is improved. The Productivity of the unit is varying from its lower value of (0.903 kg/m2.day) to a higher value of (6.47 kg/m2.day). These values are obtained for one meter square of solar air heater area. Gained Output Ratio (GOR) values range from a minimum of (0.082) to a maximum of (0.572). It is reasonable when compared to ones in literature for the water-heated HDH units.

Influence of active water stream, irradiance, ambient temperature and wind speed on the efficiency of Fresnel lens based two stage PVT system

Influence of wind speed, mass flow rate of water, irradiance and ambient temperature on concentrated Photovoltaic thermal module equipped with linear FL as Primary Optic Element and Convex Lens as Secondary Optic Element have been investigated in this study. Influence of these parameters on module performance in terms of thermal efficiency and electrical efficiency are also examined during investigation. The thermal efficiency and electrical efficiency without consideration of parameters was found to be 14.3% and 51.2 % respectively. With consideration of above mentioned four parameters, the results reveal that electrical efficiency of 17.2% and thermal efficiency of 55.3% can be achieved for designed setup. Thus, there is 20% and 8% increase in electrical efficiency and thermal efficiency respectively. The electrical efficiency increases with increase in fluid flow rate, wind speed and irradiance. Electrical efficiency decreases with increase in ambient temperature. The thermal efficiency increases with increment in water mass flow rate, irradiance and ambient temperature. However, with increase in wind speed, the thermal efficiency decreases.

NUMERICAL AND EXPERIMENTAL INVESTIGATION FOR HYBRID PHOTOVOLTAIC/THERMAL COLLECTOR SYSTEM IN DUHOK CITY

The study deals with the experimental and numerical investigation of the hybrid Photovoltaic-thermal solar collector system in Duhok city during seven months and includes different measurements of temperatures, water mass flow rate, wind velocity, and solar intensity. A one-dimensional mathematical model is used to simulate the transient processes with constant thermo-physical properties and heat transfer coefficients. The energy conservation equations are solved using implicit finite difference method. The numerical and experimental results showed satisfactory agreement with an error (2.36%) between two thermal efficiencies. The results include the estimation of the electrical and thermal energy, thermal, electrical and overall efficiency. The highest overall efficiency of PV/T collector occurs in May 2019 with value (72.01%) and the lowest value in January 2019 is obtained as (63.1%). The cooling method leads to an increase in the electrical efficiency to about 3% as compared with PV solar collector system.

Numerical Study of Erosion due to Online Water Washing in Axial Flow Compressors

Recent year witnessed an increasing interest in online water washing technique since it allows to minimize compressor performance losses in the time interval between two off-line washing. However, the washing capability and the related erosion risk depend on several parameters such as the injection duration, the droplet size, the spray angle, the water mass flow rate and the injector positions. The influence of such parameters on the washing capability and erosion rate is analysed. Results are discussed with reference to number of impacts, wetted surface, capture efficiency, accumulated energy and erosion. The numerical simulation is performed with ANSYS Fluent in which a new water droplet erosion model, introduced in previous papers, is here included as a User Defined Function. The discussion provides useful information for prescribing the injector characteristics and the water washing procedure with the aim of minimizing the erosion risk.