Nurullah Kayaci

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.

Long Term Simulation of Horizontal Ground Heat Exchanger for Ground Source Heat Pump

The earth is an energy resource which has more suitable and stable temperatures than air. Ground Source Heat Pumps (GSHPs) were developed to use ground energy for residential heating. The most important part of a GSHP is the Ground Heat Exchanger (GHE) that consists of pipes buried in the soil and is used for transferring heat between the soil and the heat exchanger of the GSHP. Soil composition, density, moisture and burial depth of pipes affect the size of a GHE. There are plenty of works on ground source heat pumps and ground heat exchangers in the literature. Most of the works on ground heat exchangers are based on the heat transfer in the soil and temperature distribution around the coil. Some of the works for thermo-economic optimization of thermal systems are based on thermodynamic cycles. GHEs is commonly sized according to short time (one year or less) simulation algorithms. Variation of soil temperature in long time period is more important and, therefore, long term simulation is required to be assure the performance of the GSHP system. In this study, long time (10 years) simulation for parallel pipe GHE of a GSHP system was performed numerically with dynamical boundary conditions. In the numerical study ANSYS CFD package was used. This package uses a technique based on control volume theory to convert the governing equations to algebraic equations so they can be solved numerically. The control volume technique works by performing the integration of the governing equations about each control volume, and then generates discretization of the equations which conserve each quantity based on control volume. Thermal boundary conditions can be defined in four different types in ANSYS Fluent: Constant heat flux, constant temperature, convection-radiation and convection. In this study, periodic variation of air temperature boundary at upper surface condition is applied, the lateral and bottom surface of the solution domain are defined as adiabatic wall type boundary condition; the pipe inner surface is taken as wall with a constant heat flux. In order to provide the periodic variation of air temperature boundary at upper surface condition a User Defined Function (UDF) was written and interpreted in ANSYS Fluent. Likewise, a UDF was also written to give constant heat flux intermittently for the pipe inner surface.Constant heat flux of 10, 20, 30 W per unit length of pipe used for calculations. Effects of distance between pipes and thermal conductivity on temperature distribution in the soil were investigated. Heat transfer in the soil is time dependent three dimensional heat conduction with dynamical boundary conditions. Temperature distribution in soil were obtained and storage effect of the soil has also been investigated. An optimization methodology based on long term simulation of GHE was suggested.Copyright

Effects of Surface Conditions for Different Climatic Zones in Turkey on Temperature Distribution in the Soil

Ground Heat Exchangers (GHEs) are an important parts of Ground Source Heat Pump (GSHP) systems and its dimensions and burial depth should be calculated using an effective method considering surface conditions. Particularly, the cost of the assembly of GHE affects the choice of these systems. For efficiency of the GSHP system, the heat extracted from or dissipated to the soil should not be changed by time for longer period runs of GSHP systems. Typical values for Coefficient of Performance (COP) of Ground Source Heat Pumps (GSHPs) are up to 8 while it is 4 of air source heat pumps. Soil composition, density, moisture and burial depth of pipes affect the size of a GHE. The burial depth and the distance between the pipes are important for sizing the GHE. Because of the complexity of the boundary conditions, a numerical study was conducted to investigate the effects of time-dependent external weather conditions, burial depth, soil thermal properties and extracted heat from soil on temperature distribution in the soil. Heat transfer in the soil is time dependent three dimensional heat conduction with dynamical boundary conditions. The GHEs consist of pipes buried in the soil and are used for transferring heat between the soil and the heat exchanger pipes of GSHP. Numerical analysis performed in ANSYS software using a UDF for dynamical boundary conditions. In order to provide the periodic variation of air temperature boundary at upper surface condition was written a User Defined Function (UDF) and it can be interpreted in ANSYS Fluent. Likewise, a UDF was also written to give constant heat flux intermittently for the pipe inner surface. Temperature distribution in soil were obtained for different climatic zones and burial depths of 1–2 m. It was seen that the surface conditions of different climatic zones has great importance up to a critical value of burial depth.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

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

Numerical Investigation of the Single Phase Forced Convection Heat Transfer Characteristics of Nanofluid Flowing in Circular and Noncircular Tubes

Thermal conductivity is an important parameter that expresses the heat transfer performance of a heat transfer fluid. Due to their low thermal conductivity, conventional heat transfer fluids (e.g. water, oil, ethylene glycol mixtures) restrict the enhancement of performance and compactness in heat exchangers used in the electronic, automotive, and aerospace industries. Nanofluids are functional liquid suspensions including particles that are smaller than 100 nm. These smaller sized particles allowed forming uniform and stable suspensions. The most well-known nanoparticles are Al2O3, CuO, TiO2, each of which is used, together with the base fluids of water and ethylene glycol, in the experimental work of many researchers. Across the range of particle sizes and types of base fluids, the enhancement of thermal conductivity has been achieved under all experimental conditions with these nanoparticles. The nanofluids provide higher heat transfer enhancement than existing techniques. With some improved properties, they have extensive potential application for concentrating heat transfer performance in a variety of systems. Forced convection flows of nanofluids containing of water with TiO2 and AI2O3 nanoparticles in circular and noncircular tubes with constant wall temperature are investigated numerically in this paper. A single-phase numerical model having three-dimensional equations is solved with either constant heat flux or temperature dependent properties to determine the hydrodynamics and thermal behaviors of the nanofluid flow by means of a CFD program for the water flow in circular and noncircular tubes. An intensive literature review on the determination of the physical properties (k, μ, ρ, Cp) of nanofluids is given in the paper. The software package ANSYS Fluent was employed in the numerical study. Investigated tubes were plotted in the SolidWorks program and were imported to ANSYS Geometry. After the investigated tubes were imported to ANSYS Geometry, they were forwarded for meshing in the ANSYS Meshing program. The mesh influences the accuracy, convergence, and speed of the solution. Furthermore, the time required to create a mesh model often represents a significant portion of the time required to acquire results from the solutions; this means that the better and more automated the meshing tools, the better the solution. The numerical model is validated by means of a CFD program to compare the experimental smooth tube data as a case study and it is also solved in the CFD program for noncircular tubes as a simulation study. Velocity, temperature and pressure distributions are shown in the paper. Morever, the values of experimental and numerical are compared with each other in terms of convective heat transfer coefficients and pressure drops. Besides this, the effects of the presence of nanofluids and noncircular tubes on the heat transfer characteristics are investigated in detail.Copyright

Design and Numerical Analysis of a Vehicle Armour Against Bullets and Shaped Charges

Armored vehicles are used to carry soldiers and machinery in combat environment in safety. As a result of the experience of a military force involved in the missions in combat areas, armoring of military-vehicle should be durable against bullet impacts and shaped charge explosions. These vehicles should provide required protection of soldiers and equipment against penetration and explosion and also have good mobility (moveable capability). In order to achieve this, the vehicle must be lightweight. The most suitable material should be selected and the most suitable geometry should be designed to achieve this goal. In this study; different material combinations and different geometries are simulated using the finite element method and smooth particle hydrodynamics. Different caliber bullets are impacted into armor and their penetration affects are observed. Shaped charge explosive also used. As a result of this simulation, the material combinations are optimized to achieve the best armor.Copyright

Thermo-Economical Optimization of Ground Source Heat Pump With Horizontal Ground Heat Exchangers for a Heating Season in Istanbul: A Case Study

A ground-source heat pump (GSHP) system has three major components: a heat pump, an earth connection and an interior heating or cooling distribution system. The most important part of a ground source heat pump (GSHP) is the ground heat exchanger (GHE) that consists of pipes buried in the soil and is used for transferring heat between the soil and evaporator of the ground source heat pump. There are plenty of works on ground source heat pumps and ground heat exchangers in the literature. Most of the works on ground heat exchangers are based on the heat transfer in the soil and temperature distribution around the coil. Some of the works for thermo-economic optimization of thermal systems are based on thermodynamic cycles. In this paper, it was carried out that the thermo-economic optimization of a ground source heat pump system with horizontal ground heat exchangers operating in heating mode in Istanbul in Turkey. The monthly heat loads of a villa for every heating months were worked out by using TS825® program. Also, average soil temperature has been calculated according to soil surface temperatures taken from Turkish State Meteorological Service belonging to last ten years. An objective function was defined based on heating capacity, initial investment and operating costs of ground source heat pump (GSHP). Then, the effects of the soil thermal conductivity, burial depth and variation of soil temperature on the objective function were investigated. Also, variation of COP value was carried out for burial depth and different condensation temperatures monthly.Copyright

The Design of a Yacht-Type Chiller

This study mentioned a yacht-type chiller design working between 50 and 200 kW. Ventilation and air conditioning systems on a yacht provide the conditions necessary for the proper internal medium for the staff, passengers, cargo, machinery, and auxiliary systems. The conditions of the internal medium in a yacht are controlled by air conditioning, heating, cooling, humidification and dehumidification processes. Ventilation and air conditioning systems are primarily simple, reliable, and must be practical. Also, the ability to control noise and vibration caused by these systems and the maintenance costs should be lower. The power of the total air-conditioning and ventilation is calculated to be 105 kW for 15 rooms on a yacht. For the system to work efficiently, rather than a single system to 105 kW, 35 kW at 3 units, the establishment of a chiller system is preferred. The Mollier diagram of the system (refrigerant R-407C) is produced to accept an evaporation temperature of 1 °C (minimum) and a condensation temperature of 45 °C (maximum), and then the values of power, efficiency, and COP are calculated. By taking the design flow velocity in pipes of 12 m/s pipe diameter, the calculation is done for 3 basic lines (suction, discharge, and liquid). According to the calculated data, the suitable components were selected and assembled to establish a yacht type chiller. Necessary security checks were done for the system to work correctly, and finally, the system was ready for installation.© 2012 ASME

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

In today’s world, the efficient use of energy is very important due to short of energy sources. In order to use energy efficiently, some methods/devices have been developed recently. One of them is heat recovery systems which are used for energy saving in the Heating Ventilating and Air Conditioning applications. Air handling units (AHUs) equipped with heat recovery system can be used applications for energy saving. Not only this paper presents information about rotary heat exchangers which is one of the air to air heat recovery systems but also it investigates their effects to system when they are used in an application. In the study, a shopping centre, which is located in the capital city of Turkey, is taken consideration. The shopping centre has an air handling unit having 54567 m3/h fresh air flow rate, 640 kW heating and 41 kW cooling capacity. Calculations are performed for AHU of the shopping centre both equipped with rotary heat exchanger and without rotary heat exchanger. In order to compare performance of AHUs, annual energy saving, initial investment cost, annual operating expenses, payback time and profit parameters are calculated for each month. According to the results, heating battery power in the heating season and cooling battery power in the cooling season is significantly decreased by using heat recovery system and total annual energy saving is calculated as