Ktisis Cyprus University of Technologyhttps://ktisis.cut.ac.cyThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Mon, 20 Jan 2020 07:40:14 GMT2020-01-20T07:40:14Z50101Impact of fluid streams on horizontal wallshttps://ktisis.cut.ac.cy/handle/10488/5170Title: Impact of fluid streams on horizontal walls
Authors: Dias, Frederic; Lazari, Lazaros; Dias, Frederic; Lazari, Lazaros
Abstract: The flow of a stream coming out of a pipe and hitting a horizontal wall is considered. Both cases of rising and falling flows are studied. First, for the rising flow, depending on the length of the wall L and the Froude number F, the wall can either divert the stream or lead to its detachment. The problem is reformulated using conformal mappings and the resulting problem is then solved by a collocation Galerkin method. A particular form is assumed for the solution, satisfying Bernoulli's equation on the free surfaces at certain discrete points. The resulting equations are solved by Newton's method. Solution profiles are presented for particular values of F and the question of the lift exerted on the wall is addressed. Then, the falling flow case is studied in the presence of a horizontal wall of infinite length. Depending on the elevation H of the pipe relative to the horizontal wall and F, the flow can either leave the pipe tangentially or detach from the edge of the pipe. Results are presented showing either a tangential departure from the pipe and no squeezing, or a tangential departure from the pipe followed by squeezing of the liquid. Finally, the cases of flows in the presence of stagnation points are discussed.
Sat, 01 Jan 2011 00:00:00 GMThttps://ktisis.cut.ac.cy/handle/10488/51702011-01-01T00:00:00ZPressure-driven and other flows in microstructure optical fibres for microfluidicshttps://ktisis.cut.ac.cy/handle/10488/3437Title: Pressure-driven and other flows in microstructure optical fibres for microfluidics
Authors: Florides, Georgios A.; Lazari, Lazaros; Komodromos, Michael; Dias, Frederic; Kalli, Kyriacos; Christodoulides, Paul; Koutsides, Charalambos
Abstract: Microfluidics are important micro-scale devices that can be used to manipulate very small volumes of fluids on the order of nano- to femto-liters. The control and sorting of nano-particles is a primary goal using this technology. There is particular interest in the use of microstructure optical fibers for the transfer of fluids, whereby the guided light interacts with a fluid in the region of the air-hole structure. We study the fluid transport capabilities of microstructure fibers with cross sections containing circular or elliptical holes, considering the effects of flow rates, fluid viscosity, and the channel diameter. The role of heat flux is considered in relation to the fluid characteristics. We solve the time-dependent Navier-Stokes equations and the convection-diffusion equation. This work is of importance as one cannot assume that the flow dynamics in microstructure fibres will be the same as conventional micro-fluidic channels. Through the study of the heat transfer, for pressure-driven and other flows and for low Reynolds numbers, we confirm anticipated behaviour of the fluids in the micro-channel structure.
Mon, 01 Apr 2013 00:00:00 GMThttps://ktisis.cut.ac.cy/handle/10488/34372013-04-01T00:00:00ZEffect of the ground properties on the fluid temperature of Geothermal Heat Exchangershttps://ktisis.cut.ac.cy/handle/10488/3439Title: Effect of the ground properties on the fluid temperature of Geothermal Heat Exchangers
Authors: Florides, Georgios A; Lazari, Lazaros; Christodoulides, Paul; Messaritis, Vassilios; Theofanous, Elisavet
Abstract: Ground heat pumps (GHP) are very efficient devices that use the ground in order to
operate. GHPs reach a high coefficient of performance (COP), which is between 3 and 6 as
opposed to 1.75–2.5 for air-source heat pumps. Actual COP of a geothermal system that
includes the power required to circulate the fluid through the ground heat exchangers (GHE)
can be lower than 2.5. The setup costs are higher than for conventional systems, but the
difference is usually paid back in energy savings in 3–10 years. The ground heat exchangers
are made of polyethylene or polypropylene tubes transferring heat from or to the ground from
the heat pump.
This study describes the modeling, based on the convection-diffusion equation, of two
types of GHEs, vertical or horizontal, and examines the effect of the properties of the ground
on the fluid temperature circulating in them. For this purpose two simulation models were
constructed one for each type of HE and the FLEXPDE simulation software was used for
running the models. The large variety of ground types of Cyprus, ranging from volcanic to
sedimentary, were used for measuring the thermal properties and initial temperature.
It is shown that a nearly linear relation exists between the the initial ground temperature and the mean fluid temperature of a vertical HE, under the conditions examined.
Usually it is mentioned that a vertical HE is more efficient than a horizontal HE because, in
the cooling mode, the ground temperature below about 10 m is always cooler that the top
layer temperature, which means that the horizontal HE is embedded in hotter ground. The
results here show that this is not always so: a horizontal HE will produce a hotter outlet fluid
than a vertical one if the center to center (c-c) tube distance is kept equal to that of the
vertical HE. If the c-c distance is increased enough then the horizontal HE may produce a
lower temperature than the vertical one. This can be done easily because the width of the
trench that the horizontal HE is placed in can be increased as it is on the surface, while the
borehole diameter cannot. It is also shown that there is a critical c-c distance of the horizontal
HE, in the examined case being about 0.7 m, after which no observable effect will result on
the mean fluid temperature.
Finally, a comparison is made between various layer materials in order to study the
effect of the ground type on vertical and horizontal HEs. For this purpose the thermal
diffusivity of the ground is varied and is shown that diffusivities above 16 m2
/s are not important for the vertical HE. Below that value diffusivity is very important because the
lower the value the greater the temperature of the HE. It is observed that an increase of temperature of about 6.5°C corresponds to a diffusivity value of 3.4 m2
/s. For the horizontal
HE the critical diffusivity value is about 9 m2
/s, while a lower value of 3.4 m2
/s will produce a temperature difference, in the mean fluid, of about 3.5°C.
Tue, 01 Jan 2013 00:00:00 GMThttps://ktisis.cut.ac.cy/handle/10488/34392013-01-01T00:00:00ZModeling of Geothermal Heat Exchangershttps://ktisis.cut.ac.cy/handle/10488/3363Title: Modeling of Geothermal Heat Exchangers
Authors: Florides, Georgios A.; Lazari, Lazaros; Christodoulides, Paul; Messaritis, Vassilios; Theofanous, Elisavet
Abstract: Heat pumps (HP) attain higher efficiencies and save energy when they
are coupled to ground heat exchangers (GHEs). For this reason we
compare, by modeling, the efficiency of the vertical and horizontal
GHEs which are two basic types. The modeling of the vertical heat
exchanger is represented by two tube lines of 100 m in length,
embedded in four different types of ground with an additional bottom
base. The horizontal heat exchanger consists of four tube lines, 50 m in
length each, embedded in three ground layers. The simulation results of
the vertical GHE are validated by comparison to a measured set of data
showing very good agreement. Further simulations with the vertical GHE show, as expected, that when the initial ground temperature rises
the mean temperature of the heat exchanger fluid increases as well in a
linear relationship. For a 50-hour continuous operation, the inlet and
outlet fluid temperatures are computed for certain ground temperatures.
Comparisons between the horizontal and vertical GHEs reveal that
under the same operating conditions and center-to-center distances of
the tubes, the vertical heat exchanger keeps a much lower mean GHE show, as expected, that when the initial ground temperature rises
the mean temperature of the heat exchanger fluid increases as well in a
linear relationship. For a 50-hour continuous operation, the inlet and
outlet fluid temperatures are computed for certain ground temperatures.
Comparisons between the horizontal and vertical GHEs reveal that
under the same operating conditions and center-to-center distances of
the tubes, the vertical heat exchanger keeps a much lower meanGHE show, as expected, that when the initial ground temperature rises
the mean temperature of the heat exchanger fluid increases as well in a
linear relationship. For a 50-hour continuous operation, the inlet and
outlet fluid temperatures are computed for certain ground temperatures.
Comparisons between the horizontal and vertical GHEs reveal that
under the same operating conditions and center-to-center distances of
the tubes, the vertical heat exchanger keeps a much lower mean temperature because the initial ground temperature at the buried tube
depth is always higher than that of the vertical GHE. Because of this
observation one would assume that the vertical GHE is more efficient
than the horizontal. Instead, in a proper design, one could increase the
distance between the tube centres and in this way decrease the mean
temperature of the tube fluid. Simulations, for a 50-hour continuous temperature because the initial ground temperature at the buried tube
depth is always higher than that of the vertical GHE. Because of this
observation one would assume that the vertical GHE is more efficient
than the horizontal. Instead, in a proper design, one could increase the
distance between the tube centres and in this way decrease the mean
temperature of the tube fluid. Simulations, for a 50-hour continuous temperature because the initial ground temperature at the buried tube
depth is always higher than that of the vertical GHE. Because of this
observation one would assume that the vertical GHE is more efficient
than the horizontal. Instead, in a proper design, one could increase the
distance between the tube centres and in this way decrease the mean
temperature of the tube fluid. Simulations, for a 50-hour continuous operation and 24°C initial ground temperature, show that the mean operation and 24°C initial ground temperature, show that the mean fluid temperature can stay below that of the vertical GHE if the center-to-center distance of the tubes increases to 1 .
Tue, 01 Jan 2013 00:00:00 GMThttps://ktisis.cut.ac.cy/handle/10488/33632013-01-01T00:00:00ZFalling streams hitting horizontal walls in the presence of stagnation pointshttps://ktisis.cut.ac.cy/handle/10488/5387Title: Falling streams hitting horizontal walls in the presence of stagnation points
Authors: Dias, Frederic; Lazari, Lazaros; Dias, Frederic; Lazari, Lazaros
Sat, 01 Jan 2011 00:00:00 GMThttps://ktisis.cut.ac.cy/handle/10488/53872011-01-01T00:00:00ZGroundwater flow and Ground Heat Exchangershttps://ktisis.cut.ac.cy/handle/10488/13381Title: Groundwater flow and Ground Heat Exchangers
Authors: Lazari, Lazaros; Aresti, Lazaros; Florides, Georgios A.; Christodoulides, Paul
Abstract: The flow of groundwater in multiple ground layers can play a significant role on the cooling
or heating of vertical heat columns and Ground Heat Exchangers (GHEs), and hence on the
construction of the latter. The heat distribution over time is described by the general heat
transfer equation based on the energy balance. Thus the three-dimensional conservation of
the transient heat equation for an incompressible fluid is applied in COMSOL Multiphysics.
Heat transfer in porous media, Darcy’s velocity and seepage velocity are introduced by taking
typical values of hydraulic conductivity, along with average borehole surface temperatures on
every ground layer. The model parameters are validated against experimental values and
multiple boreholes are examined. Although the key for an overall capital cost reduction for a
GHE is known to be the borehole length, the numerical results here indicate that using the
groundwater available, construction of shallow GHE systems can be achieved with an
increase of the coefficient of performance (COP).
Sat, 01 Apr 2017 00:00:00 GMThttps://ktisis.cut.ac.cy/handle/10488/133812017-04-01T00:00:00ZClimatic warming and CO2 concentrationhttps://ktisis.cut.ac.cy/handle/10488/3988Title: Climatic warming and CO2 concentration
Authors: Florides, Georgios A.; Lazari, Lazaros; Christodoulides, Paul; Messaritis, Vassilios
Wed, 01 Jan 2014 00:00:00 GMThttps://ktisis.cut.ac.cy/handle/10488/39882014-01-01T00:00:00ZMicrofluidics in Microstructure Optical Fibers: Heat Flux and Pressure-driven and Other Flowshttps://ktisis.cut.ac.cy/handle/10488/3364Title: Microfluidics in Microstructure Optical Fibers: Heat Flux and Pressure-driven and Other Flows
Authors: Florides, Georgios A.; Kalli, Kyriacos; Lazari, Lazaros; Komodromos, Michael; Dias, Frederic; Christodoulides, Paul; Koutsides, Charalambos
Abstract: Microfluidics are important micro-scale devices that can be used to manipulate very small volumes of fluids on the order of nano- to femto-liters. The control and sorting of nano-particles is a primary goal using this technology. There is particular interest in the use of microstructure optical fibers for the heat transfer of fluids, whereby the guided light interacts with a fluid in the region of the air-hole structure.
We study the fluid transport capabilities of microstructure fibers with cross sections containing circular or elliptical holes, considering the effects of flow rates, fluid viscosity, and the channel shape. The role of heat flux is considered in relation to the fluid characteristics. Results can be obtained through the solution of the time-dependent Navier-Stokes equations and the convection-diffusion equation. This work is of importance as one cannot assume that the flow dynamics in microstructure fibers will be the same as conventional micro-fluidic channels. Through the study of the heat transfer, for pressure-driven and other flows and for low Reynolds numbers, we confirm anticipated behavior of the fluids in the micro-channel structure.
Tue, 01 Jan 2013 00:00:00 GMThttps://ktisis.cut.ac.cy/handle/10488/33642013-01-01T00:00:00ZA preliminary design of an intelligent system for the optimal utilization of renewable energy sources in buildingshttps://ktisis.cut.ac.cy/handle/10488/9275Title: A preliminary design of an intelligent system for the optimal utilization of renewable energy sources in buildings
Authors: Georgiou, Giorgos; Christodoulides, Paul; Kalogirou, Soteris A.; Florides, Georgios A.; Lazari, Lazaros
Abstract: Renewable energy is important nowadays, not only as an environmental scientific issue, but as an obligation with regard to the European regulations. As a result, scientific interest has increased towards the improvement of the different renewable energy technologies and their optimum utilization in different applications. Initially the paper reviews previous studies regarding the optimum utilization and control of the renewable energy generation installed in buildings and in general in building energy management. The final section of the paper is a novel application related to the studies reviewed; however, this application uses a new and simple way to minimize the energy consumption levels through linear programming optimization.
Wed, 01 Jun 2016 00:00:00 GMThttps://ktisis.cut.ac.cy/handle/10488/92752016-06-01T00:00:00ZOverview of energy storage technologies and a short-term storage application for wind turbineshttps://ktisis.cut.ac.cy/handle/10488/9269Title: Overview of energy storage technologies and a short-term storage application for wind turbines
Authors: Christodoulides, Paul; Marouchos, Christos; Kalogirou, Soteris A.; Florides, Georgios A.; Lazari, Lazaros; Argyrou, Maria C.
Abstract: The spreading of renewables has become stronger due to the increased air pollution. On the other hand, the penetration of renewable energy technologies causes major problems to the stability of the grid. Along with the fluctuations of the renewable energy technologies production, storage is important for power and voltage smoothing. Thus, storage technologies have gained an increased attention considering the distributed generation. This paper presents an up to date comprehensive overview of energy storage technologies. It incorporates characteristics and functionalities of each storage technology, as well as their advantages and drawbacks compared with other storage technologies. A demonstration of a short-term energy storage application in a gridconnected small wind turbine is given. The circuit is operated at the maximum power point (MPP), succeeding its connection and synchronization with the low-voltage grid. It also utilizes the energy generated by the wind turbine when a fault appears for a short period of time, through the supercapacitor.
Fri, 01 Jan 2016 00:00:00 GMThttps://ktisis.cut.ac.cy/handle/10488/92692016-01-01T00:00:00Z