Ali Celen

Forced Convective Heat Transfer of Nanofluids - A Review of the Recent Literature

The forced convection of fluids has been investigated by numerous researchers, both experimentally and numerically. A good understanding of characteristics of nanofluid flowhas thoroughly been investigated in these studies. Since the nanoparticles behave more like a single-phase fluid than a solid–liquid mixture, it is assumed that nanofluids are ideally suited in the applications as their usage causes little or no penalty in pressure drop. In recent years, many researchers have tried to fill the gaps on this subject in the literature. To meet the demand for improving the performance of heat transfer equipment, re-examination of the individual components is considered to be essential. The addition of the nanoparticles to the base fluid is one of the significant issues for the optimal performance of heat transfer systems. This paper reports on most of the forced convective heat transfer literature occurring both in-tubes and in-channels regarding the use and preparation of nanofluids. The peer reviewed papers published in citation index journals up to 2012 have been selected for review in the paper. Classification of the papers has been performed according to the publication years. The critical information on the theoretical, experimental and numerical works is presented comprehensively for each paper.

A Theoretical Comparative Study on Nanorefrigerant Performance in a Single-Stage Vapor-Compression Refrigeration Cycle

The topic of nanofluid heat transfer is certainly of interest to the heat transfer community. Nanorefrigerants are a type of nanofluids that are mixtures of nanoparticles and pure refrigerants. This paper focuses on five different nanorefrigerants with Al2O3 nanoparticles and their pure fluids: R12, R134a, R430a, R436a, and R600a. The coefficient of performance (COP) and compressor work for various evaporation and condensation temperatures are investigated. A method is developed to estimate the performance characteristics of nanorefrigerants in the refrigerant cycles for the nonsuperheating/subcooling case and superheating/subcooling case. The enthalpy of nanorefrigerants is obtained through the density. The validation process of the proposed method was accomplished with the available data in the literature. The results indicate that COP is enhanced by adding nanoparticles to the pure refrigerant and maximum values obtained using the R600a/Al2O3 mixture.

A Focus on the Literature Review of Nanorefrigerants

The heat transfer performance of various thermal devices can be augmented by active and passive techniques. One of the passive techniques is the addition of nanoparticles had the size of 1 and 100 nanometers to the common heat transfer so that the thermal transport properties of the prepared suspension called nanofluid will be enhanced compared to the base fluid. Nanorefrigerants as a special type of nanofluids which are mixtures of nanoparticles and refrigerants have a wide range of applications in diverse fields such as refrigeration, air conditioning systems and heat pumps. In this study, the missing points on this new method are also indicated regarding the lack of studies on the determination of physical properties of nanorefrigerants and the flow of nanoparticles.Copyright

A Critical Review on the Numerical Methods of Two-Phase Flows

The two-phase flow processes play a significant role in the heat transfer processes in the chemical and power industry, including in nuclear power plants. This study is a critical review on the determination of the heat transfer characteristics of pure refrigerants flowing in vertical and horizontal tubes. The authors’ previous publications on this issue, including the numerical analyses, are summarized here. The lengths of the vertical and horizontal test sections varied between 0.5 m and 4 m countercurrent flow double-tube heat exchangers with refrigerant flowing in the inner tube and cooling water flowing in the annulus. The measured data are compared to numerical predictions based on the solution of the artificial intelligence methods and CFD analyses for the condensation and evaporation processes in the smooth and enhanced tubes. The theoretical solutions are related to the design of passive containment cooling systems (PCCS) in simplified water boiling reactors (SWBR). A genetic algorithm (GA), various artificial neural network models (ANN) such as multilayer perceptron (MLP), radial basis networks (RBFN), generalized regression neural network (GRNN), and adaptive neuro-fuzzy inference system (ANFIS), and various optimization techniques such as unconstrained nonlinear minimization algorithm-Nelder-Mead method (NM), non-linear least squares error method (NLS), and Fluent CFD program are used in the numerical solution. It is shown that the heat transfer characteristics of laminar and turbulent condensing and evaporating film flows such as heat transfer coefficient and pressure drop can be predicted by means of numerical analyses reasonably well if there is a sufficient amount of reliable experimental data. Regression analysis gave convincing correlations, and the most suitable coefficients of the proposed correlations are depicted as compatible with the large number of experimental data by means of the computational numerical methods. Dependency of the output of the ANNs from various numbers of input values is also shown for condensing and evaporating flows.Copyright

Refrigerated Railroad Car Design for Shipping Frozen Meat Using Alternative Refrigerants

Refrigerated railroad cars, known as reefers, are railroad rolling stock designed to carry perishable freight at specific temperatures. They are insulated boxcars that keep the cargo at a regulated temperature. As soon as railroad cars had the capability to keep a load cold, the potential growth of the meat, dairy, fruit and brewery industries became nearly unlimited. In this paper, a cold-room system in a railroad car was investigated as a case study to illustrate the use of the sample model. The change of substitute refrigerants and insulation thickness of the cold rooms’ refrigeration system was also considered in the model. The coefficient of performance (COP), refrigerant flow rate and capacities of each component of the refrigeration system were calculated for the refrigerants R12, R22, R134a, R290, R410A, R430A, R431A, R436A, R507A, R600a as part of this analysis. As a result of the energy analyses, all of the substitute refrigerants have a slightly lower COP and require higher compressor work than R12 and R22 for a condensation temperature of 40°C. The frozen meats have an evaporation temperature of −25°C related to the preserved product in the case studies. Influences of the insulation thickness of the wall on evaporator capacity and outside temperature of various cities on the condenser are also studied.Copyright

Smoke Control of a Car Park by Means of CFD Analyses Using Jet Fans

Analysis and interpretation of daily emission ventilation with fire ventilation systems in indoor parking lots coupling with jet fans have been done by a CFD program. The ventilation of an eight-story parking lot in Istanbul is carried out and the investigation for a simplified one-story of this system is also considered. The placement of jet fans has been identified on the basis of eliminating dead jones in the passage with the help of air flow analysis. Therefore, the present study may be of great importance as both the fire and smoke can be evacuated effectively by using the optimal position of jet fans in the parking lots to maintain a less polluted atmosphere inside.Copyright

A Critical Review on the Determination of Convective Heat Transfer Coefficient During Condensation in Smooth and Enhanced Tubes

Heat exchangers using in-tube condensation have great significance in the refrigeration, automotive and process industries. Effective heat exchangers have been rapidly developed due to the demand for more compact systems, higher energy efficiency, lower material costs and other economic incentives. Enhanced surfaces, displaced enhancement devices, swirl-flow devices and surface tension devices improve the heat transfer coefficients in these heat exchangers. This study is a critical review on the determination of the condensation heat transfer coefficient of pure refrigerants flowing in vertical and horizontal tubes. The authors’ previous publications on this issue, including the experimental, theoretical and numerical analyses are summarized here. The lengths of the vertical and horizontal test sections varied between 0.5 m and 4 m countercurrent flow double-tube heat exchangers with refrigerant flowing in the inner tube and cooling water flowing in the annulus. The measured data are compared to theoretical and numerical predictions based on the solution of the artificial intelligence methods and CFD analyses for the condensation process in the smooth and enhanced tubes. The theoretical solutions are related to the design of double tube heat exchangers in refrigeration, air conditioning and heat pump applications. Detailed information on the in-tube condensation studies of heat transfer coefficient in the literature is given. A genetic algorithm (GA), various artificial neural network models (ANN) such as multilayer perceptron (MLP), radial basis networks (RBFN), generalized regression neural network (GRNN), and adaptive neuro-fuzzy inference system (ANFIS), and various optimization techniques such as unconstrained nonlinear minimization algorithm-Nelder-Mead method (NM), non-linear least squares error method (NLS), and Ansys CFD program are used in the numerical solutions. It is shown that the convective heat transfer coefficient of laminar and turbulent condensing film flows can be predicted by means of theoretical and numerical analyses reasonably well if there is a sufficient amount of reliable experimental data. Regression analysis gave convincing correlations, and the most suitable coefficients of the proposed correlations are depicted as compatible with the large number of experimental data by means of the computational numerical methods.Copyright

Effect of the Heat Recovery System to the Performance of Air Handling Unit of a Workplace

This study investigated heat recovery systems and its effects on energy saving. Heat recovery systems are used for energy saving in heating, ventilating and air conditioning applications. In the study, a workplace equipped with and without heat recovery system was taken into consideration. The workplace has an air handling unit (AHU) which has 25,000 m3/h fresh air flow rate, 414 kW heating and 356 kW cooling capacities. AHU uses mixture air including %50 rooms air and mixes in mixing filter section. As a heat recovery system, rotary type heat exchanger was selected and its performance was investigated. Annual energy saving, initial investment cost, annual operating expenses, payback time and profit parameters were calculated to compare two air handling units for each month. According to the results, cooling battery power in the cooling season and heating battery power is reduced with the usage of rotary type heat exchanger. It is obvious that presence of the heat recovery systems enhances performance of the air handling units.Copyright

Thermodynamic Analysis of Solar Heat Pump Assisted Heating Systems

In this study, a system has been developed which can be used for heating buildings by utilizing solar energy, which is free and abundant in our country, with the aim of minimizing fossil based fuel consumption especially at wintertime and this system’s efficiency has been analyzed mathematically with numerical methods.First of all, in this study the performance of a solar heat pump system is examined. The city of Antalya is chosen for this study which is in the southern region of Turkey with an abundant source of solar energy. The usability of solar energy throughout the year is examined and its application areas in building heating with the utilization of heat pumps are determined. In addition an auxiliary system with an alternative energy source is considered in case solar energy is insufficient by itself.In order to determine all of the above, energy analysis of the system is performed. The advantages of the solar heat pump heating system are studied considering the amount of energy saved and the efficiency of the system. Thermodynamic and thermoeconomic analysis has been carried out in order to determine how effectively the solar energy is utilized by the system and the advantages of solar heat pump.Copyright

Performance Comparison of Various VRV Systems for a Bank Application

In heating and cooling applications, VRV (Variable refrigerant volume) systems can be used instead of air handling units because of their low energy consumption. In this study, a bank which is kept 20°C and 24°C inside temperature heating and cooling seasons respectively was taken into consideration in order to compare different VRV systems such as VRV system for heating only mode and VRV system supported with heat pump mode device. The heat loss from bank was calculated by considering heat loss from walls, air alteration and heat sources such as human, light, devices used in the space. The outdoor integrated heating and cooling capacity calculations were performed by means of commercial software for seven different climate regions in Turkey. R410a was selected as the refrigerant of investigated VRV systems because of its low ODP (Ozone Depletion Potential) and GWP (Global Warming Potential) values and actual common usage. Effect of outside air temperature, evaporation temperature of refrigerant and outdoor unit location (such as high level, same level and lower level) on the performance of VRV systems was considered in calculations. The calculations are performed in order to clarify of effect of outdoor unit level. Payback time is calculated as 4.5 months for usage of VRV system instead of air handling unit.Copyright