Ş. Özgür Atayılmaz

Optimum distribution of thermal insulation material for constant insulation material volume or a given investment cost

Most of the recent studies based on the alternative energy sources and efficient use of energy due to the rapidly exhausting fossil energy sources. Insulation is one of the solutions applied to decrease heat losses or gains. Present studies require tedious iterations to achieve a solution for optimum distribution of thermal insulation material for the volumes confined with environments at different temperatures such as cold storages. In this study, a general analytical solution for the optimum distribution of thermal insulation material is given for the volumes confined with environments at different temperatures for a given investment cost or constant insulation material volume. Analytical solution technique given in this study eliminates the need for tedious iterations or using a specialized computer software, and can also be combined with economic analysis models to include effects of economical parameters such as fuel price, interest rate, etc. A case study is also given for explanation and application of suggested methodology for a room confined with environments at different temperatures. It is shown that optimum distribution of insulation material easily achieved by one step analytical solution technique.

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

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

Application of Artificial Neural Networks to Predict Heat Transfer From Buried Pipe for Ground Source Heat Pump Applications

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. Design of GSHP systems in different regions of US and Europe is performed using data from an experimental model. However, there are many more techniques including some complex calculations for sizing GHEs. An experimental study was carried out to investigate heat transfer in soil. A three-layer network is used for predicting heat transfer from a buried pipe. Measured fluid inlet temperatures were used in the artificial neural network model and the fluid outlet temperatures were obtained. The number of the neurons in the hidden layer was determined by a trial and error process together with cross-validation of the experimental data taken from literature evaluating the performance of the network and standard sensitivity analysis. Also, the results of the trained network were compared with the numerical study.Copyright

Effects of the Nano Particle Concentration and Type on Natural Convection Heat Transfer From Horizontal Concentric Cylinder Systems Cooling in the Ambient Air

Natural convection heat transfer from horizontal concentric cylinders is studied experimentally and numerically. Concentric cylinder was formed with two cylinders made of copper, and the annulus was filled with water. Concentric cylinder system was located in the ambient air and the inner horizontal cylinder was kept at a constant temperature. Experimental study was carried out in a conditioned room which can be maintained at a stable required value and inside a sufficiently designed test cabin. The inner copper cylinder surface temperatures varied between 30 °C–50 °C and ambient temperature was 20 °C. On the basis of the experimental data average Nusselt numbers for the air side of the concentric cylinders were calculated and compared with numerical results. The effective thermal conductivity of the annulus was calculated by using the experimental data and numerical solution results and compared. Besides, heat transfer enhancement in horizontal annuli has been investigated for different nano particle concentrations. Water based nano fluid containing Al2O3 nano particles have been used. Isotherms and streamlines are presented in the annulus and the air side. Heat transfer rates under steady state conditions for different nano particle concentrations were compared and heat transfer enhancement was determined.Copyright

Determination of Two-Phase Flow Void Fraction of R600a in a Horizontal Smooth Tube

In this paper, the void fraction of alternative refrigerant R600a flowing inside horizontal tube is determined by means of an experimental technique, well known correlations in the literature and a generalized neural network analysis. The horizontal tube is made from smooth glass tubing of 4 mm inner diameter. The test runs are done at average saturated condensing temperatures between 30 and 40 °C while the average qualities and the mass fluxes are between 0.45–0.91 and 68.5–138.1 kg m-2s-1 respectively. The flow regime determination inside the tube is performed by means of sight glasses placed at the inlet and outlet sections of the test section, used for in-tube condensation tests, virtually. An image processing technique, performed by means of a high speed camera, is used to determine the void fractions of stratified and annular condensing flow of R600a experimentally. The void fractions are determined using relevant measured data together with 11 different void fraction models and correlations reported in the open literature analytically. Artificial neural network (ANN) analysis is developed to determine the void fractions numerically. For this aim, mass flow rate, average vapor quality, saturation temperature, liquid and vapor densities, liquid and vapor dynamic viscosities and surface tension are selected as the input parameters, while the void fraction is selected as the output.Three-layer network is used for predicting the void fraction. The number of the neurons in the hidden layer was determined by a trial and error process evaluating the performance of the network and standard sensitivity analysis. The measured void fraction values are found to be in good agreement with those from ANN analysis and correlations in the literature. It is also seen that the trained network are more predictive on the determination of void fraction than most of the investigated correlations.Copyright