Secondary Heat Transfer Research Articles

Parameter estimation applied to the heat transfer characterisation of Scraped Surface Heat Exchangers for food applications

A parameter estimation approach was applied to characterise the heat transfer of Scraped Surface Heat Exchangers (SSHEs) specifically designed for the food industry. It is difficult to apply the data available in the literature to SSHEs, due to the specificity of each product, thermal treatment and geometrical configuration, making the thermal design of these apparatuses critical. Therefore, it appears to be more useful to assess the methodology used to derive a proper heat transfer correlation than to assess the form of the heat transfer correlation itself, as the correlation often cannot be transferred to other heat exchangers, even those that belong to the same class.This study enabled successful and robust estimation of the heat transfer correlation for the product side Nusselt number and the external side heat transfer coefficient; this approach differs from Wilson plot methods, as no assumption is made regarding the functional dependence of the external side heat transfer coefficient.The procedure was validated through application to both synthetic data and experimental data acquired from a coaxial SSHE pilot plant for the treatment of highly viscous fluid foods.The procedure was optimised with the aid of sensitivity and uncertainty analysis, which provided considerable insight into the problem.The application to synthetic data demonstrated that under typical operating conditions, areas of insensitivity to certain parameters are present. The application to the experimental data acquired under both heating and cooling conditions confirmed that the measured values of the overall heat transfer coefficient can be used to estimate the secondary fluid heat transfer coefficient, as well as the power law dependence of the internal fluid Nusselt number on the rotational Reynolds number and the Prandtl number together with the multiplicative constant. The uncertainty analysis provided the confidence intervals associated with each estimated parameter, thereby enabling the quality and robustness of the resulting heat transfer correlations to be determined.

Heat Transfer Characteristics of Passive Condensers for Reactor Containment Cooling

Passive condensers that are based on gravitational force do not require pumps or blowers to move fluid and, hence, are considered as safety measures in nuclear power plant containment cooling. In this paper vertical tube passive condensers with the tube inside condensation and cooled by a secondary pool of water are discussed as applied to reactor containment heat removal system. Series of experiments were carried out on scaled separate effects test facilities with vertical single tube and four tube bundle condensers. The results of experimental data on the rate of condensation heat transfer, the effects of noncondensable, and parametric effects, such as pressure, tube diameters and lengths, obtained from single tube and tube bundle passive condenser are presented. The data on the tube bundle indicated larger condensation heat transfer due to enhanced heat transfer in the secondary cooling. For tube bundle condensation, the turbulent mixing on the secondary pool side decreases the temperature difference between pool water and condenser tube outer wall, causing an increase in secondary heat transfer. Theoretical models on tube condensation based on heat and mass analogy and boundary layer models were developed. The data were compared with theoretical models and good agreement was observed between theoretical predictions and single tube condensation heat transfer data. For the tube bundle condenser, the theoretical model agreed with data when the secondary side enhancement in heat transfer due to turbulent mixing in the flow boiling was included. Practical heat transfer correlations are presented using available data on passive condensers for reactor application.

Studies on the solubility of hydrogen in molten Pb83Li17 eutectic alloy

Lead–lithium eutectic (Pb87Li17) alloy is a candidate material to be used as a secondary tritium breeder, neutron multiplier and heat transfer agent in the fusion reactor. The tritium thus produced in the alloy may be soluble or appear as a new phase of lithium-tritides and/or lead-tritides, which eventually affect the performance of Pb83Li17 eutectic. Therefore, solubility of tritium in the alloy at the operating conditions of the fusion reactor is a subject matter of investigation. Tritium being the isotope of hydrogen behaves more or less similar to the hydrogen. In the present investigation the solubility of hydrogen in the Pb83Li17 has been investigated as a function of temperature and pressure. It was found that, hydrogen solubility in the Pb87Li17 alloy is almost constant above 350 °C. Hydrogen solubility increases with increase of temperature up to 400 °C. Hydrogen solubility is 120 ppm at 400 °C and 800 Torr hydrogen pressure. The solubility decreases on further rise in temperature from 400 °C. However, at all the temperatures hydrogen solubility increased with increase of partial pressure of hydrogen.

Improving high temperature heat capture for power generation in gasification plants

As part of the gasification process, hot syngas is cooled to low temperatures for gas cleaning by generating high pressure steam in a radiant heat exchanger. The steam, in turn, is used to generate electric power. However, the very large temperature difference that exists between the syngas and generated high pressure steam results in significant losses during the heat transfer process. These losses can be reduced, and hence the overall plant efficiency improved, by choosing materials and working fluids that reduce the temperature difference in the syngas heat exchanger. A set of alternatives to high pressure steam is liquid metals or molten salts. These liquids can be kept at a pressure equal to the syngas pressure, reducing the tensile hoop stress in the heat exchanger tubes. Since the strength of steel decreases with temperature, reducing the hoop stress allows tubes (and the liquids inside them) to reach higher temperatures. The liquid metals or molten salt can then be used in a secondary heat transfer loop to transfer heat to a steam generator. This paper describes a cost-effective radiant heat exchanger that can be used to transfer heat from high temperature syngas to these alternative heat transfer fluids, and also quantifies the increase in overall IGCC plant efficiency by doing so. A capital cost assessment shows a small payback time and improved revenue is achieved by using this technology, making this a realistic option for gasification processes.

Numerical investigation of thermal–hydraulic characteristics in a steam generator using a coupled primary and secondary side heat transfer model

A coupled primary and secondary side heat transfer and thermal phase change model is used to investigate the thermal–hydraulic characteristics of a steam generator (SG) at Daya Bay Nuclear Power Plant (DBNPP). The simulation results reasonably reveal the ununiform boiling behavior in the secondary hot and cold legs. Vapor velocity is slightly higher than that of water and the corresponding slip-ratio first increases rapidly before gradually decreasing in the secondary hot leg and cold leg regions, a result which agrees with predictions using the drift-flux model. Cross-flow energy, which accounts for flow-induced vibration (FIV) at the U-bend tubes, is determined with the aid of localized thermal–hydraulic distributions, and the resulting FIV damage is predicted to be most severe at 0.35m on the cold leg side and −0.2m on the hot leg side of the U-bend region, respectively. These FIV damage predictions agree with measured plant data for the prototypical SG, showing that this model can provide the use information to improve thermal–hydraulic characteristics and help alleviate FIV damage in a SG.

Development of the Simulation Software for Secondary Cooling Heat Transfer in Beam Blank Continuous Casting Process Based on FLUENT

A 2D mathematical Model for heat transfer during solidification in the secondary cooling was established on researching the process of beam blank continuous casting. Meanwhile, the boundary conditions of heat transfer was subdivided into four parts, which is the radiation, spray cooling, heat conduction through roller and water accumulated evaporation in secondary cooling zoon, and improved the results accuracy. Moreover, heat transfer simulation software was designed and developed by a 2D mathematical Model based on FLUENT. The secondary development language UDF (User Define Function) and Scheme (a dialect of LISP) of FLUENT were used during the development. The software provided a platform for researching on the solidification and optimization of a secondary cooling system for a caster machine of beam blank.

Molten Salt Mixture Properties (KF-ZrF4 and KCl-MgCl2) for Use in RELAP5-3D for High-Temperature Reactor Application

Molten salt coolants are being investigated as primary coolants for a fluoride high-temperature reactor and as secondary coolants for high-temperature reactors such as the next-generation nuclear plant. This work provides a review of the thermophysical properties of candidate molten salt coolants for use as a secondary heat transfer medium from a high-temperature reactor to a processing plant. The molten salts LiF-NaF-KF, KF-ZrF4, and KCl-MgCl2 were considered for use in the secondary coolant loop. The thermophysical properties necessary to add the molten salts KF-ZrF4 and KCl-MgCl2 to RELAP5-3D were gathered for potential modeling purposes. The properties of the molten salt LiF-NaF-KF were already available in RELAP5-3D. The effect that the uncertainty in individual properties had on the Nusselt number was evaluated. This uncertainty in the Nusselt number was shown to be nearly independent of the molten salt temperature.

Modelling an active magnetic refrigeration system: A comparison with different models of incompressible flow through a packed bed

Abstract Active Magnetic Regenerative refrigeration system (AMR) is an environmentally attractive refrigeration alternative to vapour compression plants. A magnetic refrigerator is composed of a regenerator that is a solid packed bed made of a magnetic material and a secondary heat transfer fluid flowing in the porous matrix. The secondary fluid can be a liquid (water or water anti-freezing mixture). Modelling is a particularly cost-effective method for studying, designing and optimizing a magnetic refrigeration system. In this paper a comparison between three different models to simulate the thermo-fluidodynamic behaviour of an AMR cycle is carried out. Each model simulates both the magnetic material and the secondary fluid of an AMR operating in conformity with a Brayton regenerative cycle. In the reported models Gd x Dy 1−x alloys are the constituent materials for the regenerator over the temperature range 260–280 K. The heat transfer medium is a water-glycol mixture (50% by weight).With these models, the refrigeration capacity, the power consumption and consequently the coefficient of performance of the cycle can be predicted.

Investigation on droplet impingement erosion during steam generator tube failure accident

Abstract Droplet impingement erosion is one of the reasons causing the secondary heat transfer tube damage in case of the heat transfer tube failure in the steam generator of sodium-cooled fast reactor. This paper has been dedicated to investigate the impingement phenomena, pressure load and the damage rate by impingement. Single droplet impingement on the rigid wall was first simulated with MPS-AS (moving particle semi-implicit method for all speeds) method for water and sodium. The shockwave propagation during the impingement has been well captured. The pressure load by water impingement can be characterized with Heymann's correlation (1968) . A correlation for pressure load by sodium impingement is proposed based on the water hammer theory. The new correlation shows to be consistent with the simulation result. For prediction of erosion rate by sodium droplet impingement, Heymann's empirical correlations (1979) were extended by including the effects of liquid properties, i.e. density, sound speed and viscosity. The extended correlations show similar prediction accuracy to the original ones.

Numerical computation for parallel plate thermoacoustic heat exchangers in standing wave oscillatory flow

A simplified computational method for studying the heat transfer characteristics of parallel plate thermoacoustic heat exchangers is presented. The model integrates the thermoacoustic equations of the standard linear theory into an energy balance-based numerical calculus scheme. Details of the time-averaged temperature and heat flux density distributions within a representative domain of the heat exchangers and adjoining stack are given. The effect of operation conditions and geometrical parameters on the heat exchanger performance is investigated and main conclusions relevant for HX design are drawn as far as fin length, fin spacing, blockage ratio, gas and secondary fluid-side heat transfer coefficients are concerned. Most relevant is that the fin length and spacing affect in conjunction the heat exchanger behavior and have to be simultaneously optimized to minimize thermal losses localized at the HX-stack junctions. Model predictions fit experimental data found in literature within 36% and 49% respectively at moderate and high acoustic Reynolds numbers.

Comparative study of a novel heat pipe photovoltaic/thermal collector and a water thermosiphon photovoltaic/thermal collector

To solve the freezing problems associated with the water thermosiphon photovoltaic/ thermal collector (PV/T) system, and decrease the temperature difference between the photovoltaic cells, a novel heat pipe (PV/T), which uses R600a as a working fluid, is proposed in this article. Comparative performance tests with the water thermosiphon PV/T system are conducted. The temperature difference between the PV cells of the novel heat pipe system is obviously smaller than that in the water thermosiphon. Thus, the photovoltaic conversion efficiency of the novel heat pipe system is improved at 9.5 per cent compared to 8.9 per cent in the water thermosiphon system. However, because of secondary heat transfer and large frictional resistance, the photothermal conversion of the novel heat pipe PV/T system is less efficient than that of the water thermosiphon system. Preliminary results show that the photothermal efficiency of the novel heat pipe PV/T system is 23.8 per cent, while that of the water thermosiphon system is 33.0 per cent. Although the two systems exhibit different photovoltaic efficiency and photothermal efficiency levels, from the perspective of the second law of thermodynamics, both systems possess almost the same exergy efficiency of approximately 10.3 per cent.

Heating Performance of Zeotropic Mixture and Temperature Glide Matching with Secondary Fluid

In order to decrease the heat rejection pressure of pure CO2 refrigerating system and meanwhile have sustainable environmentally benefits for working fluild, several natural zeotropic CO2/Propane mixtures are applied to heat pump system to investigate the heating performance. The heat rejection pressure of CO2/Propane mixtures is reduced when the mass fraction of carbon dioxide is decreased. Under the given conditions, there is an optimum range of mass fraction of CO2/Propane of which the mixtures have superior heating coefficient of performance. The temperature glide characteristics of CO2/Propane mixtures at heating mode was also researched to analyze the heat transfer efficiency. Both a sufficient heat exchange area and a good working fluid’s temperature gradient matching with that of the secondary heat transfer fluid contribute to a higher heat transfer efficiency of heat exchanger.

Experimental investigation of a vapor compression heat pump used for cooling and heating applications

The present work aims at evaluating the performance characteristics of a vapor compression heat pump (VCHP) for simultaneous space cooling (summer air conditioning) and hot water supply. In order to achieve this objective, a test facility was developed and experiments were performed over a wide range of evaporator water inlet temperature (14:26 °C) and condenser water inlet temperature (22:34 °C). R134a was used as a primary working fluid whereas water was adopted as a secondary heat transfer fluid at both heat source (evaporator) and heat sink (condenser) of the heat pump. Performance characteristics of the considered heat pump were characterized by outlet water temperatures, water side capacities and coefficient of performance (COP) for various operating modes namely: cooling, heating and simultaneous cooling and heating. Results showed that COP increases with the evaporator water inlet temperature while decreases as the condenser water inlet temperature increases. However, the evaporator water inlet temperature has more effect on the performance characteristics of the heat pump than that of condenser water inlet temperature. Actual COP of cooling mode between 1.9 to 3.1 and that of heating mode from 2.9 to 3.3 were obtained. Actual simultaneous COP between 3.7 and 4.9 was achieved.

Determining the optimum cyclic operation of adsorption chillers by a direct method for periodic optimal control

Adsorption refrigeration systems provide a sustainable possibility to reduce the environmental impact of refrigeration and air-conditioning as they allow for sources of otherwise unused excess waste heat to be reused for cooling purposes. Adsorptive cooling is a discontinuously operated cycling process, and it is well known that the determination of an optimal cycling time yielding maximum cooling power is a key to the design of an efficient mode of operation. The optimal cycle time however strongly depends on operating conditions such as ambient air temperature, available heat source temperature, desired target cooling temperature, and achievable volume flow rates of the secondary heat transfer circuits. In this contribution, we apply a direct method for periodic optimal control to optimize two-bed adsorption chillers. We present a first principles dynamic model of the underlying thermal process. We show that direct methods for periodic optimal control allow for quick and reliable computation of optimal cycle times for a given set of parameters. Contrary to pre-existing methods, fast computation times and guaranteed optimality of the solutions computed by our approach makes it viable to extensively study the simulated optimal cyclic operation of two-bed adsorption chillers under a wide range of varying conditions.

Self-Contained, Oscillating Flow Liquid Cooling System for Thin Form Factor High Performance Electronics

A self-contained, small-volume liquid cooling system for thin form-factor electronic equipment (e.g., blade server modules) is demonstrated experimentally in this paper. A reciprocating water flow loop absorbs heat using mesh-type microchannel cold plates and spreads it periodically to a larger area. From there, the thermal energy is interchanged via large area, low pressure drop cold plates with a secondary heat transfer loop (air or liquid). Four phase-shifted piston pumps create either a linearly or radially oscillating fluid flow in the frequency range of 0.5–3 Hz. The tidal displacement of the pumps covers 42–120% of the fluid volume, and, therefore, an average flow rate range of 100–800 ml/min is tested. Three different absorber mesh designs are tested. Thermal and fluidic characteristics are presented in a time-resolved and a time-averaged manner. For a fluid pump power of 1 W, a waste heat flux of 180 W/cm2(ΔT=67 K) could be dissipated from a 3.5 cm2 chip. A linear oscillation flow pattern is advantageous over a radial one because of the more efficient heat removal from the chip and lower hydraulic losses. The optimum microchannel mesh density is determined as a combination of low pump losses and high heat transfer rates.

Energetic performances of a refrigerating loop using ice slurry

The consideration of environmental constraints in production, transport and distribution of cold energy resulted in reconsidering the practices of installations dimensioning in particular. Their containment led to the development of secondary refrigerants such as ice slurries to store, transport and distribute the cold energy. These heat transfer fluids should have good thermophysical properties, giving high transport capability, high heat transfer ability as well as low pressure drops. The use of ice slurries can lead to lower flow rates and smaller pumping power compared to single phase fluid. The purpose of the presented work is to study the distribution network of indirect cold systems thanks to a model allowing the evaluation of the influence of various parameters on the operating behaviour of the installation. The available domain for the use of secondary heat transfer fluid (whether in their single phase or two phase form) is determined considering the best design from an energetic point of view. Because of the essential role of the fluid distribution between the production site and consumers, we focus our study on pressure drops and pumping power due to the fluid flow in cooling loops. For each investigated case, the minimum consumption power is obtained with the two phases (solid-liquid) heat transfer fluid (ice slurry).

Characterizing convective heat transfer using infrared thermography and the heated-thin-foil technique

Convective heat transfer, due to axial flow fans impinging air onto a heated flat plate, is investigated with infrared thermography to assess the heated-thin-foil technique commonly used to quantify two-dimensional heat transfer performance. Flow conditions generating complex thermal profiles have been considered in the analysis to account for dominant sources of error in the technique. Uncertainties were obtained in the measured variables and the influences on the resultant heat transfer data are outlined. Correction methods to accurately account for secondary heat transfer mechanisms were developed and results show that as convective heat transfer coefficients and length scales decrease, the importance of accounting for errors increases. Combined with flow patterns that produce large temperature gradients, the influence of heat flow within the foil on the resultant heat transfer becomes significant. Substantial errors in the heat transfer coefficient are apparent by neglecting corrections to the measured data for the cases examined. Methods to account for these errors are presented here, and demonstrated to result in an accurate measurement of the local heat transfer map on the surface.

Modeling of condensation heat transfer for a PRHRS heat exchanger in a SMART-P plant

A theoretical model using a heat and mass transfer analogy and a simple model using Lee and Kim's [Lee, K.-Y., Kim, M.H., 2008a. Experimental and empirical study of steam condensation heat transfer with a noncondensable gas in a small-diameter vertical tube. Nucl. Eng. Des. 238, 207–216] correlation were developed to investigate steam condensation in the presence of a noncondensable gas inside a vertical tube submerged in pool water. Rohsenow's correlation was used to consider the secondary pool-boiling heat transfer. Both models were assessed with the experimental data of Oh and Revankar [Oh, S., Revankar, S.T., 2005a. Investigation of the noncondensable effect and the operational modes of the passive condenser system. Nucl. Technol. 152, 71–86; Oh, S., Revankar, S.T., 2005b. Effect of noncondensable gas in a vertical tube condenser. Nucl. Eng. Des. 235, 1699–1712; Oh, S., Revankar, S.T., 2005c. Complete condensation in a vertical tube passive condenser. Int. Commun. Heat Mass Trans. 32, 593–602; Oh, S., Revankar, S.T., 2005d. Analysis of the complete condensation in a vertical tube passive condenser. Int. Commun. Heat Mass Trans. 32, 716–727; Oh, S., Revankar, S.T., 2006. Experimental and theoretical investigation of film condensation with noncondensable gas. Int. J. Heat Mass Trans. 49, 2523–2534; Oh, S., Gao, H., Revankar, S.T., 2007. Investigation of a passive condenser system of an advanced boiling water reactor. Nucl. Technol. 158, 208–218] for low pressure and Kim [Kim, S.J., 2000. Turbulent film condensation of high pressure steam in a vertical tube of passive secondary condensation system. Ph.D. dissertation, Korea Advanced Institute of Science and Technology] for high pressure, which were obtained from in-tube steam condensation with air in the pool water. These models predicted the data of Oh and Revankar well, but they slightly underestimated the data of Kim. The design of the Passive Residual Heat Removal System (PRHRS) condensation heat exchanger was evaluated with the theoretical model at real operating conditions (e.g., secondary pool-boiling, high system pressure). The PRHRS condensation heat exchanger designed was estimated to remove sufficiently the remaining heat in a reactor during a major accident.

Performance of R290 and R1270 for R22 applications with evaporator and condenser temperature variation

In this study, therma l performance of two hydrocarbon refrigerants of R290 and R1270 was measured in an attempt to substitute R22. They wer tested in a heat pump bench tester of 1 ton capacity with a hermetic rotary compressor. Water and water/glycol mixture were employed as the secondary heat transfer fluids in the test bench. All tests were conducted under the same external conditions simulating three different air-conditioning and heat pumping conditions. Test results show that the coefficient of performance of these hydrocarbon refrigerants is up to 11.5% higher than that of R22 under all conditions. Refrigeration capacity of R290 is up to 8.2% lower than that of R22 under normal air-conditioning and heat pumping conditions. Under extremely cold temperature conditions, however, the capacity of R290 is 5% higher than that of R22. On the other hand, the capacity of R1270 is similar to that of R22 under all conditions. Compressor discharge temperatures of these hydrocarbons are reduced by 14–31°C as compared to R22. The amount of charge is reduced up to 58% as compared to R22. Overall, these hydrocarbons provide good performance with reasonable energy savings without any environmental problems and thus can be used as long-term alternatives for residential air-conditioning and heat pumping applications.

Cooling performance of several CO 2/propane mixtures and glide matching with secondary heat transfer fluid

This paper presents the cooling performance of several CO 2/propane mixtures measured in air-conditioning test rig at several conditions. The discharge pressure of CO 2/propane mixtures is reduced with increasing mole fraction of propane and their reduced values coincide approximately with the circulation concentrations of propane. Since propane is the refrigerant having a higher refrigerating effect and a much lower vapor density than CO 2, adding propane to CO 2 improves the system efficiency and reduces the cooling capacity. The temperature glide effect of CO 2/propane mixtures on the cooling performance was analyzed based on the experimental data. To utilize the temperature glide effect successfully, a sufficient heat exchange area is required, and the temperature gradient of refrigerant must be similar to that of secondary heat transfer fluid. It is better the temperature change of refrigerant can prevent pinching with that of the secondary heat transfer fluid.