Derya Burcu Özkan

Experimental Investigation of the Defrosting Process on Domestic Refrigerator Finned Tube Evaporators

Frost formation is an important problem for household refrigerator and air conditioning equipment manufacturers. When frost accumulates on the evaporator surface, it acts as a thermal insulator and reduces heat flow. Therefore, frost negatively affects evaporator performance. The purpose of this study is to decrease energy consumption and increase the efficiency of the defrosting process. In the first part of the experiment, frost formation on a no-frost refrigerator evaporator at real operating conditions was investigated. The ambient temperature was maintained at a constant 23°C. It was observed that when the evaporation temperature reaches –35°C, the frost formation on the evaporator exhibits a rather dense structure that is unlike the needle-type structure observed at higher temperatures. In the second part of the experiment, the defrosting process was observed with an endoscopic camera, and the initial melting points were investigated. The experiment revealed that although the heater density is higher on the lower rows, the frost on the higher rows of the evaporator melts faster. On the theoretical side, we prepared an analytical model that calculates the melting time of the frost on the fin. The experimental and theoretical results are within 5%.

CFD and Experimental Analysis of a Falling Film outside Smooth and Helically Grooved Tubes

Simultaneous heat and mass transfer are investigated in a falling film outside grooved and smooth tubes. A numerical analysis of the helically trapezoidal-grooved and reference smooth tube was performed in the computational fluid dynamics program “Ansys Fluent 14.” The three-dimensional model drawings in the x, y, and z coordinates are used, and the effects of the falling film outside the helically grooved tube on the surface temperature and surface heat transfer coefficient are determined. The average surface temperature, heat transfer coefficient, and Nu values are determined experimentally for a constant heat flux. An uncertainty analysis and Nu correlation for the grooved tube are also provided in this study. The Reynolds number varied between 50 and 350 for the falling film and between 1500 and 3500 for air. Using a computational fluid dynamics (CFD) analysis for the reference smooth tube, the experimental results are validated within 2–12% difference. The experimental results are also within 6–13% of the grooved tubes.

Comparison of EGR ratios determined by four different methods for electronic re-circulation gate control

In this study, low pressure systems have been applied for exhaust gas recycling (EGR), which is applied in certain operating conditions. Induction air, exhaust gas flow, amount of exhaust gas added to the induction air, amount of carbon dioxide (CO2) in the exhaust gas, and amount of carbon dioxide in the induction air mixed with the exhaust gas, have been measured and the EGR ratio has been determined using four different calculation methods. The obtained results have been compared, and it has been noted that all the four methods have given similar results, and the deviation among them has been noted to be 1% at most. Deviations among the results have been explained. In the concluding section, a relation has been formed between the variables chosen as basic parameters, electronic re-circulation gate control, and variables utilised in EGR ratio determination methods.

Theoretical study of R32 to replace R410A in variable refrigerant flow systems

ABSTRACT In recent years, global warming has reached critical levels and this has caused a gradual decrease in the use of refrigerants. In the 1997 Kyoto Conference, it was announced that after 2011, use of refrigerants which have higher global warming potential (GWP) of 150 will be restricted. Hence, it was foreseen that R410A, with a GWP of 2088, which has been used in residential heating, ventilating and air conditioning systems, will be prohibited in the near future. This study investigates the thermodynamic performance using R32, which is a potential alternative because of its low GWP, and R410A in variable refrigerant flow (VRF) systems. As a result of theoretical analysis, it was observed that the coefficient of performance of R32 is 5% higher in heating mode and 6% higher in cooling mode than that of R410A. Considering all of this, it was confirmed that R32 can be used in VRF systems as a replacement for R410A in terms of performance.

The Effect of Insulation on Energy Savings and Costs for the Current Building Inventory in Turkey

The current building inventory in Turkey includes a large number of uninsulated buildings. By insulating these buildings, the required heating energy is decreased, and significant energy savings are achieved. Within the country, the standard “Thermal Insulation Requirements for Buildings” was placed into effect in buildings in the year 2000 and divided the country into four climatic regions. The optimum insulation thickness in buildings changes as a function of the region, type of insulation material and fuel. In this study, the implemented insulation thickness according to the “Thermal Insulation Requirements for Buildings” standard is calculated for the four different regions of Turkey for expanded polystyrene insulation material. The current building inventory in Turkey was taken from the State Institute of Statistics (Prime Ministry, Republic of Turkey). Energy savings, costs and payback period are calculated by the P1-P2 method for the current building inventory, using XPS insulation in all buildings and natural gas as the fuel for the heating energy requirements. The results are shown in the figures for the four different climatic regions, and the effect of the current building inventory distribution around the country is determined.© 2012 ASME

Heat Transfer Analysis of Internally Grooved Copper Tube R404-A Evaporators

Internally grooved copper tubes are used extensively in HVAC applications, direct expansion batteries and air or water cooled heat exchangers. The advantage of internally grooved copper tubes in evaporator and condenser units is an increase in the refrigerant-side heat transfer coefficient. When an internally grooved tube heat exchanger and a smooth-tube heat exchanger with the same dimensions are compared, the overall heat transfer coefficient and convective heat transfer coefficient are found to increase in different ratios. In addition to this difference, the refrigerant side pressure is found to be a function of the groove geometry, pitch space and choice of refrigerant. In this study, which is different from previous studies in the literature performed using single internally grooved tube condensers and evaporators, refrigerant R404-A is studied in the internally grooved tube evaporator. The heat transfer in the evaporator described here is 30% better than that observed in a conventional smooth-copper-tube evaporator. In the internally grooved tube, the internal surface area is 68% larger than that inside the smooth reference tube. As a result, the convective heat transfer coefficient inside the internally grooved tube is found to be lower than that in the smooth tube.Copyright

NOx Formation of Co-combustion of Sweet Sorghum – Lignite (Orhaneli) Mixtures in Fluidised Beds

Nitric oxide is an acid rain precursor and participates in the generation of photochemical smog through ozone production. In addition, nitrous oxide is a greenhouse gas. Thus, the reduction of emissions of nitrogen oxides is currently a major environmental issue. In this study, Sweet sorghum was chosen as the energy plant. Combustion experiments of lignite and lignite-sweet sorghum mixtures were carried out in a fluidized bed system. The fuel-feeding ratio was set such that the thermal output of the system remained constant. Addition of Sweet sorghum to the lignite reduces the pollutant concentration. The results were supported by experimental results.