Fred Gunnerson

THEORETICAL DESIGN OF THERMOSYPHON FOR PROCESS HEAT TRANSFER FROM NGNP TO HYDROGEN PLANT

The Next Generation Nuclear Plant (NGNP) will most likely produce electricity and process heat, with both being considered for hydrogen production. To capture nuclear process heat, and transport it to a distant industrial facility requires a high temperature system of heat exchangers, pumps and/or compressors. The heat transfer system is particularly challenging not only due to the elevated temperatures (up to ∼ 1300K) and industrial scale power transport (≥50 MW), but also due to a potentially large separation distance between the nuclear and industrial plants (100+m) dictated by safety and licensing mandates. The work reported here is the preliminary analysis of two-phase thermosyphon heat transfer performance with alkali metals. A thermosyphon is a device for transporting heat from one point to another with quite extraordinary properties. In contrast to single-phased forced convective heat transfer via ‘pumping a fluid’, a thermosyphon (also called a wickless heat pipe) transfers heat through the vaporization / condensing process. The condensate is further returned to the hot source by gravity, i.e. without any requirement of pumps or compressors. With this mode of heat transfer, the thermosyphon has the capability to transport heat at high rates over appreciable distances, virtually isothermally and without any requirement for external pumping devices. Two-phase heat transfer by a thermosyphon has the advantage of high enthalpy transport that includes the sensible heat of the liquid, the latent heat of vaporization, and vapor superheat. In contrast, single-phase forced convection transports only the sensible heat of the fluid. Additionally, vapor-phase velocities within a thermosyphon are much greater than single-phase liquid velocities within a forced convective loop. Thermosyphon performance can be limited by the sonic limit (choking) of vapor flow and/or by condensate entrainment. Proper thermosyphon requires analysis of both.Copyright

Design of Liquid Metal Phase Change Heat Exchanger for Next-Generation Nuclear Plant Process Heat Application

The Next Generation Nuclear Plant will most likely produce electricity and its reactor heat will be further utilized for the production of hydrogen, oil recovery from tar sands and oil shales, and other process heat applications, that will further the nations pursuit of energy independence. An intermediate heat exchanger is required to transfer heat from the Next-Generation Nuclear Plant to the hydrogen plant (or other processes) in the most efficient way possible. Phase change heat exchangers are quite attractive in this regard, as they can transfer process heat more efficiently than for the single phase due to the advantage of high-enthalpy transport that includes the sensible heat of liquid, the latent heat of vaporization, and possible vapor superheat. This paper explores the overall heat transfer characteristics and pressure drop of the phase change heat exchanger with helium as the primary and sodium as the secondary heat exchanger coolant. For a two-phase boiling regime, the convective heat transfer coefficient is based on the concept of an additive, interacting mechanism of micro- and macroconvective heat transfer. In this analysis an improved design is proposed for given conditions, so as to obtain a lower overall pressure drop and a moderate/high overall heat transfer coefficient. The analysis presented in this paper will be useful as a guide for future experimental work for Next Generation Nuclear Plant process heat transfer.

EFFECT OF MASS FLOW RATE ON THE CONVECTIVE HEAT TRANSFER COEFFICIENT : ANALYSIS FOR CONSTANT VELOCITY AND CONSTANT AREA CASE

Abstract The effect of the mass flow rate at constant velocity on the convective heat transfer coefficient of an incompressible fluid in a turbulent flow regime is presented with the help of dimensional analysis. The heat transfer coefficient decreases by ˜10% with a threefold increase in the mass flow rate under these conditions, based on the commonly used Dittus-Boelter correlation for estimation of the heat transfer coefficient. On the other hand, an increase in the heat transfer coefficient is observed if the area is maintained constant. Doubling the mass flow rate will result in a 92% increase in the heat transfer coefficient. However, there is a concomitant increase in the pressure drop, proportional to the mass flow rate raised to 0.95. The pressure drop is predicted to decrease for the constant velocity case with an inverse dependence on the mass flow rate. The pressure drop considerations may be critical in certain situations (elevation of boiling point in case of a boiling heat transfer medium), and any benefit derived from the higher heat transfer coefficient may be lost because of the higher pressure drop across the heat exchanger in the constant area case.

Dimensionless Numbers in Phase-Change Thermosyphon and Heat-Pipe Heat Exchangers

Abstract Heat pipes and thermosyphons can be very effective heat transport devices for transferring the thermal energy of the Next Generation Nuclear Plant to a hydrogen production plant and/or other process heat applications. These devices operate nearly isothermally, transporting large amounts of thermal energy with little or no temperature drop. A dimensional analysis of the thermosyphon and the heat pipe is presented in this paper. Dimensional analysis is a valuable mathematical technique useful in research work for design and conducting model tests. This analysis yielded two terms - Er and EM - particular to the operation of these devices in addition to those commonly used in many heat transfer applications. The Er term relates the latent heat of vaporization to the pressure drop across the device, while the EM term relates the latent heat of vaporization to the capillary pressure. The significance of these two terms is discussed. The universal nature of these numbers should be useful in increasing the fundamental understanding of both the thermosyphon and the heat pipe.

Theoretical Design of a Thermosyphon for Efficient Process Heat Removal from Next Generation Nuclear Plant (NGNP) for Production of Hydrogen

The work reported here is the preliminary analysis of two-phase Thermosyphon heat transfer performance with various alkali metals. Thermosyphon is a device for transporting heat from one point to another with quite extraordinary properties. Heat transport occurs via evaporation and condensation, and the heat transport fluid is re-circulated by gravitational force. With this mode of heat transfer, the thermosyphon has the capability to transport heat at high rates over appreciable distances, virtually isothermally and without any requirement for external pumping devices. For process heat, intermediate heat exchangers (IHX) are required to transfer heat from the NGNP to the hydrogen plant in the most efficient way possible. The production of power at higher efficiency using Brayton Cycle, and hydrogen production requires both heat at higher temperatures (up to 1000oC) and high effectiveness compact heat exchangers to transfer heat to either the power or process cycle. The purpose for selecting a compact heat exchanger is to maximize the heat transfer surface area per volume of heat exchanger; this has the benefit of reducing heat exchanger size and heat losses. The IHX design requirements are governed by the allowable temperature drop between the outlet of the NGNP (900oC, based on the current capabilities of NGNP), and the temperatures in the hydrogen production plant. Spiral Heat Exchangers (SHE’s) have superior heat transfer characteristics, and are less susceptible to fouling. Further, heat losses to surroundings are minimized because of its compact configuration. SHEs have never been examined for phase-change heat transfer applications. The research presented provides useful information for thermosyphon design and Spiral Heat Exchanger.

NGNP Process Heat Utilization: Liquid Metal Phase Change Heat Exchanger

One key long-standing issue that must be overcome to fully realize the successful growth of nuclear power is to determine other benefits of nuclear energy apart from meeting the electricity demands. The Next Generation Nuclear Plant (NGNP) will most likely be producing electricity and heat for the production of hydrogen and/or oil retrieval from oil sands and oil shale to help in our national pursuit of energy independence. For nuclear process heat to be utilized, intermediate heat exchange is required to transfer heat from the NGNP to the hydrogen plant or oil recovery field in the most efficient way possible. Development of nuclear reactor-process heat technology has intensified the interest in liquid metals as heat transfer media because of their ideal transport properties. Liquid metal heat exchangers are not new in practical applications. An important rationale for considering liquid metals as the working fluid is because of the higher convective heat transfer coefficient. This explains the interest in liquid metals as coolant for intermediate heat exchange from NGNP. The production of electric power at higher efficiency via the Brayton Cycle, and hydrogen production, requires both heat at higher temperatures and high effectiveness compact heat exchangers to transfer heat to either the power or process cycle. Compact heat exchangers maximize the heat transfer surface area per volume of heat exchanger; this has the benefit of reducing heat exchanger size and heat losses. High temperature IHX design requirements are governed in part by the allowable temperature drop between the outlet of NGNP and inlet of the process heat facility. In order to improve the characteristics of heat transfer, liquid metal phase change heat exchangers may be more effective and efficient. This paper explores the overall heat transfer characteristics and pressure drop of the phase change heat exchanger with Na as the heat exchanger coolant. In order to design a very efficient and effective heat exchanger one must optimize the design such that we have a high heat transfer and a lower pressure drop, but there is always a tradeoff between them. Based on NGNP operational parameters, a heat exchanger analysis with the sodium phase change is presented to show that the heat exchanger has the potential for highly effective heat transfer, within a small volume at reasonable cost.Copyright