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Applications of Multiphase Heat Transfer.Pdf 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer 1.6 Applications of Multiphase Heat Transfer Selected applications of multiphase systems in these technologies are reviewed in this section. 1.6.1 Energy Systems Including Fuel Cells and Combustors Thermal Energy Storage Figure 1.17 Schematic of latent heat thermal energy storage system. Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 1 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer The major barrier to more widespread use of solar energy is its periodic feature, i.e., it is available only during daytime, and so a heat storage device is needed to store energy and release it for use at night. The latent heat thermal energy storage system, which utilizes phase-change materials (PCMs) to absorb and release heat, is widely used for this purpose. The PCM in the thermal energy storage system is molten when the system absorbs heat, and it solidifies when the system releases heat. The advantages of the latent heat thermal energy storage system are that: a large amount of heat can be absorbed and released at a constant temperature the size of the latent heat thermal energy system is considerably smaller than its counterpart using sensible heat thermal energy storage. Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 2 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer Power and Refrigeration Cycles The condenser is a major component in power plants as well as in air conditioning units and refrigerators. The condenser converts exhaust steam/refrigerant vapor into liquid and by rejecting heat to the ambient environment. Fig. 1.18 shows a schematic diagram of a typical water-cooled condenser for a modern power plant. The cooling water flows inside the tubes and the exhaust steam condenses on the outside surfaces of the tubes. To improve the efficiency of the steam power plant, it is desirable to increase the average heat-addition temperature. One practical way to increase the temperature of feedwater entering the boiler is to incorporate a feedwater heater, which uses steam extracted from various points of the steam turbine to heat the feedwater. Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 3 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer Figure 1.18 Water-cooled condenser (Lock, 1994). Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 4 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer Figure 1.19 Feedwater heater concept. Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 5 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer Figure 1.20 Boiler (Lock, 1994). Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 6 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer Phase Change during Combustion Combustion is a chemical reaction process between a fuel (which can be solid, liquid or gas) and an oxidant to produce high-temperature gases, which can be used to generate steam in a boiler, drive a gas turbine, or melt metals in a metallurgical process. Except in cases where gaseous fuels are used, the combustion process always involves phase changes. Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 7 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer When solid fuel, such as coal, is used in the combustor, combustion occurs on the surface of the solid fuel. In order to increase the contact area between the coal and oxidant, the coal is ground into fluidized particles (very small particles that can flow with the oxidant gas) that are consumed during the combustion. Combustion of solid fuel involves gas-solid two-phase flow interaction between solid particles diffusion of oxidant near the particle surfaces conduction heat transfer in the solid particle convective heat transfer in the gas as well as chemical reactions on the particle surface that consume the solid particles Since the densities of solid fuel and oxidant are significantly different, resulting flow patterns are usually not homogeneous because the solid particles and the oxidant possess different velocities. Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 8 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer Fan Compressors Combustor Fan Stream Primary Stream Turbines Figure 1.21 Components and main flows of the turbofan engine (courtesy of GE-P&W Engine Alliance, LLC). Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 9 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer Diffuser Case Fuel Nozzle Liner Figure 1.22 Schematic of typical turbofan combustor. Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 10 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer Fuel Cells A fuel cell is an electrochemical energy device that converts the chemical energy in the fuel directly into electrical energy. It is becoming increasingly attractive alternatives to other conversion technologies, from small-scale passive devices like batteries to large-scale thermodynamic cycle engines. Unlike conventional power devices, i.e., steam turbines, gas turbines and internal combustion engines, which are based on certain thermal cycles, the maximum efficiency of fuel cells is not limited by the Carnot cycle principle. A fuel cell generally functions as follows: 1. electrons are released from the oxidation of fuel at the anode, 2. protons (or ions) pass through a layer of electrolyte, 3. the electrons are required for reduction of an oxidant at the cathode. The desired output is the largest flow of electrons possible over the highest electric potential. Although other oxidants such as the halogens have been used where high efficiency is critical, oxygen is the standard because of its availability in the atmosphere. Fuel cells typically use hydrogen, carbon monoxide or hydrocarbon fuels (i.e., methane, methanol). The hydrogen and carbon monoxide fuels may be the products of catalytically processed hydrocarbons. Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 11 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer In the first case, the oxidant at the cathode combines with electrons, which tend to circumvent the electrolyte, and becomes anions which travel through the electrolyte to the anode At the anode, the anions give up their electrons and combine with hydrogen to form water. The water, depleted fuel, and products are exhausted from the anode surface, and the depleted oxidant and products are exhausted from the cathode surface. In the second case, where the electrolyte conducts cations, the hydrogen containing fuel is decomposed electrochemically, giving up electrons and leaving hydrogen cations to travel through the electrolyte. Upon reaching the cathode the cations combine electrochemically with the oxidant and electrons, which tend to circumvent the electrolyte to form water. Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 12 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer Oxidant (e.g., Oxygen, Air….) Reactant Products (e.g., H2O, CO2 ….) & Heat Cathode Load Electrolyte e- Anode Fuel (e.g., H2, CH4, CO, CH3OH….) Figure 1.23 Fuel cell schematic. Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 13 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer There are several types of fuel cells, and they belong to either of the two cases just described. Types of anion-conducting electrolyte fuel cells are: Alkaline Fuel Cells – for example, those using potassium hydroxide molten carbonate that operates at about 650C, and Solid Oxide Fuel Cells that operates to 1000C. Cation-conducting electrolyte fuel cells include Phosphoric Acid Fuel Cells Polymer Electrolyte Membrane Fuel Cells. Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 14 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer The fuel cell stack consists of repeated interleaved structure of MEAs, GDLs and bipolar plates. It is evident that flow channels are an essential component for flow distribution in many PEMFC deigns. The flow channels in a PEM fuel cell are typically on the order of a 1 mm hydraulic diameter, which falls into the range of minichannels (with hydraulic diameters from 200 μm to 3 mm). As shown in Fig. 1.24, one channel wall is porous (gas diffusion layer); mass transfer occurs on this wall along its length. Hydrogen is consumed on the anode side along the main flow dimension in minichannels. Oxygen from air is introduced on cathode side to form water at catalyst sites at the cathode, which is transported into the minichannels through the gas diffusion layer and then is eventually removed from the cell by the gas flow and gravity if so oriented. Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 15 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer Gas Diffusion Layer Catalyst Layer Proton Exchange Membrane Bipolar Plate Bipolar Plate e- H2O H2O + e- H H+ H2 O2 H2 O2 Figure 1.24 Basic construction of a typical PEM fuel cell stack (Faghri and Guo, 2005) Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 16 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer Gas Purification Hydrogen (H2) CO2 +
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