Conceptual Design and Numerical Modeling of Prototype Counterflow Cooling Tower With Forced Draft for Geothermal Applications

Cooling towers are widely used in temperature control in industrial processes and electricity generation processes by conventional and renewable energy methods. In this paper, it is presented an integral design of a counterflow cooling tower with forced draft for geothermal applications. The conceptual design was done in SolidWorks® software and the numerical simulation of the fluid through the tower was performed in Fluent® software. In the conceptual design were made both structural and tower elements design of the counterflow tower with forced draft. Besides, it was designed a self-drive sprinkler which distributes the water flow to be cooled inside the tower. In the mathematical model the velocity and temperature profiles were analyzed under different turbulence models that allow to increase their accuracy, as a result of this, it was able to calculate the heat transfer in the boundary layer between the walls packing and circulating air inside the tower. As a consequence could be estimate the coefficient of convective heat transfer.

On Numerical Simulation of Fuel Mixed With Steam or Air to Investigate Soot and Other Emissions

Soot emissions (PM 2.5) as well as CO and NOx from industrial flares and other industrial processes or sources pose a substantial risk to human being health and the environment, and now are subject to new and tougher EPA regulations. Flaring is used widely used in many industries to dispose unwanted combustion gases by burning them as a flame. However, flaring produces significant amount of particulate matter in the form of soot, along with other harmful gas emissions. Although many experimental and numerical studies have previously been done on flames burning in a controlled condition, relatively few studies have been conducted with fuel-steam mixture. In practice, air and steam are commonly used to assist the flaring processes — control the smoke and the combustion efficiency. This study aims to investigate soot, CO and NOx emissions of turbulent diffusion methane and propane flame mixed with air or superheated steam. To study such effect numerically, the computational fluid dynamics software ANSYS Fluent 14.5 is used with non-premixed probability density function (PDF) model. The laminar flamelet is generated with automated grid refinement. For the soot generation, the Moss-Brookes soot model with Lee sub-model is considered. The combustion mechanism is developed by the authors’ research group from the combined GRI and USC mechanisms. Two types of fuel, methane and propane, are used. The amount of super-heated steam varied from four percent to twenty percent (4%, 8 %, 12%, 16%, and 20%), and the behavior of the flame is analyzed. For the baseline case, the jet has a diameter of 50.8 mm or 2 inches, and the jet velocity is kept to 1.0 m/s. A co-flow air is supplied at a velocity of 0.2 m/s. The temperature distribution of methane and propane are compared with different contents of steam or air assists. The NOx, Soot and CO yields (kg/kg) varying with steam or air percentages are also presented. The results indicate that the soot yield is dependent on fuel type strongly and the percentage of steam or air affects the soot yield differently as the fuel type varies.

Transient Response of a Cross Flow Heat Exchanger Subjected to Temperature and Flow Perturbations

The transient performance of a multi-pass cross flow heat exchanger subjected to temperature and mass flow rate perturbations, where the heat exchanger flow circuiting is neither parallel flow nor counter flow, is considered in this work. A detailed numerical study was performed for representative single-pass, two-pass, and three-pass heat exchangers. Numerical predictions were obtained for cases where the minimum capacity rate fluid was subjected to a step change in inlet temperature in absence of mass flow rate perturbations. Likewise, numerical predictions were obtained for the heat exchangers operating initially at steady state, where a step mass flow rate change of the minimum capacity rate fluid was imposed in the absence of any fluid temperature perturbations. The transient performance of this particular heat exchanger configuration subjected to these temperature and flow disturbances has not been discussed previously in the available literature. In the present study the energy balance equations for the hot and cold fluids and the heat exchanger wall were solved using an implicit central finite difference method. A parametric study was conducted by varying the dimensionless quantities that govern the transient response of the heat exchanger over a typical range of values. Because of the storage of energy in the heat exchanger wall, and finite propagation times associated with the inlet perturbations, the outlet temperatures of both fluids do not respond instantaneously. The results are compared with previously published transient performance predictions of multi-pass counter flow and parallel flow heat exchangers.

Pool Boiling Heat Transfer Enhancement of Water Using Brazed Copper Microporous Coatings

A novel, high-temperature, thermally-conductive, microporous coating (HTCMC) is developed by brazing copper particles onto a copper surface. This coating is more durable than many previous microporous coatings and also effectively creates reentrant cavities by optimizing brazing conditions. A parametric study of coating thicknesses of 49–283 μm with an average particle size of ∼25 μm was conducted using the HTCMC coating to understand nucleate boiling heat transfer (NBHT) enhancement on porous surfaces. It was found that there are three porous coating regimes according to their thicknesses. The first regime is “microporous” in which both NBHT and critical heat flux (CHF) enhancements gradually grow as the coating thickness increases. The second regime is “microporous-to-porous transition” where NBHT is further enhanced at lower heat fluxes but decreases at higher heat fluxes for increasing thickness. CHF in this regime continues to increase as the coating thickness increases. The last regime is named as “porous”, and both NBHT and CHF decrease as the coating thickness increases further than that of the other two regimes. The maximum nucleate boiling heat transfer coefficient observed was ∼350,000 W/m2K at 96 μm thickness (“microporous” regime) and the maximum CHF observed was ∼2.1 MW/m2 at ∼225 μm thickness (“porous” regime).

Theoretical Analysis of Resonance Tube End Wall Heating: Solution of Unsteady Fluid Dynamic and Energy Equations

A solution of the highly complex unsteady compressible flow field inside a cylindrical resonance tube has been obtained numerically, assuming one dimensional, viscous, and heat conducting flow, by solving the appropriate fluid dynamic and energy equations. The resonance tube is approximated by a right circular cylinder closed at one end with a piston oscillating at resonant frequency at the other end. An iterative implicit finite difference scheme is employed to obtain the solution. The scheme permits arbitrary boundary conditions at the piston and the end wall and allows assumptions for transport properties. For the example considered herein, the solution predicts a rise of 95°F in the mean end wall temperature, from 60°F to 155°F, in 14.313 milliseconds which is in good agreement with the experimentally observed values. The solution would also be valid for tapered tubes if the variations in the cross-sectional area are small. In successfully predicting the resonance tube results, an innovative simple but stable solution of unsteady fluid dynamic and energy equations is provided here for wide ranging research, development, and industrial applications in solving a variety of complex fluid flow heat transfer problems. The method is directly applicable to pulsed or pulsating flow and wave motion thermal energy transport, fluid-structure interaction heat transfer enhancement, and fluidic pyrotechnic initiation devices.

A Numerical Investigation of Nanofluids Thermal Behavior in Microchannels Under Laminar Flow Conditions

Due to large amounts of heat flux developed in electronic devices, it is essential to propose and investigate effective mechanisms of cooling them. Although microchannels filled with flowing coolant are a geometry often met in such devices, new techniques need to be developed in order to increase their effectiveness. Recent studies on nanofluids, i.e. mixtures of nanometer size particles well-dispersed in a base fluid, have demonstrated their potential for augmenting heat transfer. In the present work the 2D steady state laminar flow of different nanofluids along a microchannel is examined. It is considered that the microchannel walls receive uniform and constant heat flux. The problem’s modelling has as parameters the volume fraction of nanoparticles ranging from 0 to 5% and Reynolds number varying between 50 and 500. The results of the problem’s numerical solution are used to calculate the heat transfer coefficient, the pressure drop along the microchannel and the destroyed exergy. It is found that heat transfer is enhanced due to the presence of nanoparticles. On the contrary, pressure drops faster due to nanofluids increased viscosity leading to more pump power needed. Finally, further exergy destruction is observed when nanoparticles volume fraction increases.

Performance Analysis of a Silica Gel-Water Adsorption Cooler: Impact of Nanofluids on Cooler Performance and Size

Thermally driven chillers also known as sorption heat pumps have drawn considerable attention in recent years. They can be divided into two main categories: absorption (liquid-vapor) and adsorption (solid-vapor) systems. Even though adsorption cycles have relatively lower coefficient of performance compared to absorption cycles, however they prevail in terms of heat source, electric consumption for moving parts, crystallization etc. In order to overcome the drawback of low COP and specific cooling capacity, nanofluids, i.e. mixtures of nanometer size particles well-dispersed in a base fluid, can be used as heat transfer fluids as recent experimental and theoretical research has proved that nanofluids can exhibit a significant increase on heat transfer. In this study a two bed, single-stage adsorption chiller which utilizes the silica gel-water pair as adsorbent-refrigerant is simulated. The cooling capacity and the COP of the chiller are calculated for various cycle times. The usage of nanofluids as heat transfer fluids in the chiller evaporator and condenser and their effect on chiller performance and size is investigated. It is proved that the presence of nanofluids at different volume concentrations will enhance the cooling capacity and the COP of the adsorption chiller and therefore will lead to smaller, in terms of size, heat exchangers.

Numerical Analysis of Transient Temperature Distribution in a Partially Cooled Nuclear Fuel Rod

Critical safety studies of a nuclear power plants are often associated with inadequate and improper cooling of the reactor core or the spent fuel rods. Coolant flow over the hot nuclear fuel rods often gets stalled during major accidents resulting in high temperature levels. These elevated temperature levels can potentially melt the fuel rod material and cause the release of radioactive gases. Research activities, both numerical and experimental in nature to explore these rare but potentially catastrophic possibilities have resulted in sophisticated numerical codes capable of simulating the various post-accident scenarios. These codes, although reasonably accurate and reliable have steep learning curves and are not often very user-friendly. A fast and accurate prediction of the critical temperature conditions using popular commercially available software packages is the subject of current study. Results from this parametric study of temperature distribution over a partially cooled fuel rod carried out using ANSYS as the numerical analysis tool is reported. Nuclear fuel rods being inadequately cooled inside a stagnant pool of coolant water in an accident scenario resulting in disrupted coolant flow has been simulated. This situation can arise within the reactor (design-basis accidents) or in the waste-fuel storage (as faced in Fukushima). In these situations, the fuel rod is often left partially immersed in the coolant water resulting in immersed portion of the rod cooled by water and the exposed portion cooled by air leading to non-uniform and improper cooling of the system. Realistic dimensions and materials as in commercial nuclear fuel rod have been used in the study. Taking advantage of the symmetry, an axisymmetric radial plane sliced longitudinally has been analyzed. Variations in the tangential direction have been neglected. The heat transfer problem uses homogeneous convective boundary conditions and assumes temperature dependent thermal conductivity. The parameters varied are the coolant level and the heat generation rate inside the fuel rod. A macro to automatically capture the transients in the temperatures was written in ANSYS (a finite element package). The governing energy equations were implicitly solved using finite volume scheme in MATLAB. ANSYS results are in close agreement with those obtained using MATLAB. The centerline temperature of the fuel rod shows a sharp rise below a certain coolant level.

Flow Past Square Cylinders of Different Size With and Without Corner Modification

In many mechanical engineering applications, separated flows often appear around any object such as tall buildings, monuments, and towers are permanently exposed to wind. Similarly, piers, bridge pillars, and legs of offshore platforms are continuously subjected to the load produced by maritime or fluvial streams. These bodies usually create a large region of separated flow and a massive unsteady wake region in the downstream. The highly asymmetric and periodic nature of flow in the downstream has attracted the attention of physicists, engineers and CFD practitioners. A lot of research work is carried out for a square cylinder but flow past square cylinders with and without corner modification work is not taken up. This motivated to take up the task of flow past two different sized square cylinders, numerically simulated. A Reynolds number of 100 and 200 is considered for the investigation. The flow is assumed to be two dimensional unsteady and incompressible. The computational methodology is carried out once the problem is defined the first step in solving the problem is to construct a geometry on which the simulation is planned. Once the geometry is constructed, proper assignment of its boundaries in accordance to the actual physical state is to be done. The various boundary options that are to be set. After setting the boundary types, the continuum type is set. The geometry is discretized into small control volumes. Once the surface mesh is completed, the mesh details are exported to a mesh file, then exported to Fluent, which is CFD solver usually run in background mode. This helps to prioritize the execution of the run. The run would continue until the required convergence criterion is reached or till the maximum number of iterations is completed. Results indicate, in case of chamfered and rounded corners in square cylinder, there is decrease in the wake width and thereby the lift and drag coefficient values. The form drag is reduced because of a higher average pressure downstream when separation is delayed by corner modification. The lift coefficients of Square cylinder with corner modification decreases but Strouhal number increases when compared with a square cylinder without corner modification. Strouhal number remains same even if magnitude of oscillations is increased while monitoring the velocity behind the cylinder. Frequency of vortex shedding decreases with the introduction of second cylinder either in the upstream or downstream of the first cylinder. As the centre distance between two cylinders i.e., pitch-to-perimeter ratio is increased to 6,the behavior of the flow almost approaches to that of flow past a square cylinder of with and without modification of same condition. When the perimeter of the upstream cylinder with and without modification is larger than the downstream cylinder, the size of the eddies is always bigger in between the cylinders compared to the downstream of the second cylinder. The flow velocity in between the cylinders with and without corner modification are less compared to the downstream of the second cylinder. As the distance increases, the flow velocity in between the cylinders become almost equal to the downstream of the second cylinder. The results are presented in the form of streamlines, flow velocity, pressure distribution. drag coefficient, lift coefficient and Strouhal number.

Experimental Study on Radiant Floor Heating System

Radiant floor heating systems have become popular due to their advantages over conventional heating systems in residential, commercial and industrial spaces. They are also used for snow and ice melting and turf conditioning applications. This paper presents a general study focuses on the design of radiant floor heating systems and investigates the effect of design parameters such as pipe spacing (ranging from 4 in. to 12 in.), pipe depth (ranging from 2.5 in. to 6.5 in.) and pipe temperature (45 °C, 65 °C and 85 °C) on the performance of radiant floor heating system embedded in different mediums (air, gravel and sand). The experimental results showed that a radiant heating system with pipes embedded at a shallow burial depth and placed closer together resulted with a more desired floor temperature distribution. The average floor temperature was also higher when the piping system was embedded in an air-filled space instead of a porous medium such as gravel or sand.