Experimental research on reverse flow phenomenon in an equal-height-difference natural circulation system at low pressure

PIV-based experiments on reverse flow characteristics in an equal-height-difference passive heat removal system for ocean nuclear power plants

The current paper presents a novel experimental study on flow boiling in heat sinks with rectangular cross-section parallel microchannels fabricated by conventional powder metallurgy process. The dimensions of microchannel heat sinks are 20 mm × 20 mm. Porosities of heat sinks produced by sintering vary between 13.9% and 21.3%. A pure (nonporous) copper heat sink was also produced to compare the experimental data with each other. Channel sizes and numbers of all heat sinks used in the tests are identical. Flow boiling tests were carried out with deionized water at a flow rate of 238.2 kg/m<sup>2</sup> s and 430.32 kg/m<sup>2</sup> s. The fluid enters the heat sink at 80°C, completely liquid. The tests were carried out under constant heat flux conditions and the heat fluxes are 46.5 W/cm<sup>2</sup> and 61.5 W/cm<sup>2</sup>. Experimental studies have shown that the heat transfer coefficients in the heat sink with the highest porosity are from 28% to 33.3% higher than in the pure copper heat sink. Experimental heat transfer coefficients increase with increasing flow rate and decrease with increasing heat flux and vapor quality. On the other hand, the pressure drop values of the heat sink sample with the highest porosity were measured to be one and half times more than that of the pure copper heat sink. The imaging technique was used during the experiments. With the imaging technique, it was determined that the fluctuations in the pressure values were caused by the reverse steam flow. Additionally, the heat transfer coefficient values obtained experimentally were compared with the existing correlations in the literature. Experimental results showed that pure heat sinks are more compatible with the results given in the literature than porous heat sinks.

Chapter 7 - Instabilities in natural circulation systems

Dimethyl carbonate (DMC) synthesis from methanol and carbon dioxide in the presence of ZrO2 solid solutions and yield improvement by applying a natural convection circulation system

Using ground-source heat pumps (GSHPs) instead of fossil-fuel based energy systems can significantly reduce the building sector's considerable carbon footprint. Most GSHPs rely on borehole heat exchangers (BHEs), which use the subsurface as a heat source in winter and a heat sink in summer. Realizing the full carbon-saving potential of GSHPs requires accurate design, installation, and operation of both the heat pumps and BHEs over their 30–50-year service life. In practice, however, uncertainties in subsurface thermal properties, simplified design tools, and limited information about future energy demand often lead to operational performance falling short of design expectations. This design–operation gap prevents GSHPs from operating at maximum efficiency. The work presented in this thesis aims at bridging the gap by identifying its causes, demonstrating the benefits of optimized operation, and contributing to model development. A GSHP system with 40 BHEs serves as the case study. An extensive monitoring system provides six years of high-resolution data on fluid temperatures and flow rates for each BHE. Based on the data, four studies focusing on different aspects of the design-operation gap are conducted. In the first study, the data is used to reveal the design–operation gap for the studied BHE field. The monitored field delivers, on average, 116 % of the planned cooling energy but only 6 % of the intended heating. Moreover, the data is analyzed with a focus on two commonly overlooked factors in design practice: (i) different heat exchange between BHEs and (ii) thermal gains and losses in the horizontal pipes connecting BHE heads to underground vaults. BHE heat exchange differs by around 20 %, which is partially attributed to horizontal pipes: A BHE with a 45 m horizontal pipe has an up to 75 % higher volumetric energy yield for cooling, while the energy yield is reduced for heating by about 50 %, both during cold months. The analysis highlights the design–operation gap and underscores the need for improved models and control strategies that can account for the analyzed influencing factors. In the second study, the dataset is used to assess the predictive accuracy of four BHE models (a numerical, transfer function, resistance-capacitance, and hybrid model) through sensitivity analysis, calibration, and forward simulation. Excluding horizontal header pipes, the hybrid model best matches measured outlet temperatures over both short (one-month) and long (four-year) periods. Transfer functions and resistance-capacitance models rely more heavily on calibration, but they are easier to implement in optimization frameworks due to their simpler structure. The third study evaluates the real-world impact of BHE optimization, which is often explored through simulation but rarely tested in practice. We analyzed data of 15 months of optimized operation in the BHE field, where model predictive control selected only a subset of BHEs to operate, and compared it to standard operation, where all 40 BHEs were operated simultaneously. Optimized operation reduced pumping electricity by two-thirds and tripled BHE field performance, i.e., resulted in significant energy savings. Finally, we explored whether the thermal effect of horizontal piping can be embedded in thermal response functions, a standard model used for BHE simulation. To this end, we derived data-driven response functions for each BHE and, in parallel, simulated model-based counterparts using a physics-based model that includes horizontal pipes. The results show that the inclusion of horizontal pipes in the model results in a better alignment with the data. As expected, data-driven response functions perfectly reproduce the measured data, but only 38 % of the physics-based response functions achieved similar accuracy. The results show that data-driven methods offer both speed and accuracy but are sensitive to measurement errors and uncertainty, thus requiring validation with physical models. Together, the studies demonstrate that the analyzed BHE field can serve as a demonstrator for bridging the design-operation gap. Applying these learnings to other BHE fields demands a site-specific integration of geosciences, energy planning, uncertainty analysis, continuous monitoring, and operation optimization. Further research should explore how these methods can be incorporated into existing planning processes and standards to support more adaptive and data-informed BHE system design.

Experiments on the basic behavior of supercritical CO2 natural circulation

Natural circulation systems find extensive applications in industrial engineering systems. One of the applications is in nuclear reactor where the decay heat is removed by natural circulation of the fluid under off-normal conditions. The upcoming reactor designs make use of natural circulation in order to remove the heat from core under normal operating conditions also. These reactors employ multiple vertical fuel channels with provision of on-power refueling/defueling. Natural circulation systems are relatively simple, safe and reliable when compared to forced circulation systems. However, natural circulation systems are prone to encounter flow instabilities which are highly undesirable for various reasons. Presence of parallel channels under natural circulation makes the system more complicated. To examine the behavior of parallel channel system, studies were carried out for single-phase natural circulation flow in a multiple vertical channel system. The objective of the present work is to study the flow behavior of the parallel heated channel system under natural circulation for different operating conditions. Steady state and transient studies have been carried out in a parallel channel natural circulation system with three heated channels. The paper brings out the details of the system considered, different cases analyzed and preliminary results of studies carried out on a single-phase parallel channel system.

Chapter 3 - Governing differential equations for natural circulation systems

Experiments were performed on natural convection heat transfer from circular pin fin heat sinks subject to the influence of its geometry, heat flux and orientation. The geometric dependence of heat dissipation from heat sinks of widely spaced solid and hollow/perforated circular pin fins with staggered combination, fitted into a heated base of fixed area is discussed. Over the tested range of Rayleigh number, 3.8×106≤Ra≤1.65×107, it was found that the solid pin fin heat sink performance for upward and sideward orientations shows a competitive nature, depending on Rayleigh number and generally shows higher heat transfer coefficients than those of the perforated/ hollow pin fin ones in both arrangement. For all tested hollow/perforated pin fin heat sinks, however, the performance for sideward facing orientation was better than that for upward facing orientation. This argument is supported by observing that the augmentation factor was around 1.05–1.11, depending on the hollow pin diameter ratio, D i /D o . Meanwhile, the heat sink of larger hollow pin diameter ratio, D i /D o offered higher heat transfer coefficient than that of smaller D i /D o for upward orientation, and the situation was reversed for sideward orientation. The heat transfer performance for heat sinks with hollow/perforated pin fins was better than that of solid pins. The temperature difference between the base plate and surrounding air of these heat sinks was less than that of solid pin one and improved with increasing D i /D o

PurposeThe purpose of this paper is to optimize the configuration sizes of micro‐channel cooling heat sink using the thermal resistance network model. The optimized micro‐channel heat sink is simulated by computational fluid dynamics method, and the total thermal resistance is calculated to compare with that of thermal resistance network model.Design/methodology/approachTaking the thermal resistance and the pressure drop as goal functions, a multi‐objective optimization model was proposed for the micro‐channel cooling heat sink based on the thermal resistance net work model. The Sequential Quadratic Programming procedure was used to do the optimization design of the structure size of the micro‐channel. The optimized micro‐channel heat sink was numerically simulated by computational fluid dynamics (CFD) software.FindingsFor the heat sink to cool a chip with the sizes of L × W = 2.5 mm × 2.5 mm and the power of 8 W, the optimized width and height of the micro‐channel are 154 μm and 1,000 μm, respectively, and its corresponding total thermal resistance is 8.255 K/W. According to the simulation results, the total thermal resistance of whole micro‐channel heat sink Rtotal is 7.596 K/W, which agrees well with the analysis result of thermal resistance network model.Research limitations/implicationsThe convection heat transfer coefficient is calculated approximatively here for convenience, and that may induce some errors. Originality/value –The maximum difference in temperature of the optimized micro‐channel cooling heat sink is 59.064 K, which may satisfy the requirement for removal of high heat flux in new‐generation chips.

Study on spray cooling characteristics of dry ice particles for IGBT in high-speed trains{fr}Etude des caractéristiques de refroidissement par pulvérisation des particules de glace sèche pour l'igbt dans le train à grande vitesse

Scaling analyses for the open flashing-driven natural circulation system

To experiment 20kW OTEC, the closed-cycle type of OTEC (Ocean Thermal Energy Conversion) was designed and manufactured. R32 (Difluoromethane, CH2F2) was used as the working fluid and a temperature of heat source and heat sink is 26°C, 5°C, respectively. The semi-welded type heat exchanger is applied for the evaporator and condenser and the cycle was designed for the gross power of 20kW. In the plate arrangement of the semi-welded type heat exchanger, one channel for working fluid is welded, and another channel for seawater is sealed by gasket. In this paper, various performance evaluations and experiments were carried out as constructing subminiature pilot plant of the OTEC and compared with the results of cycle analysis. In results, gross power of the turbine shows 20.1kW and cycle efficiency is 1.91% when heat source and heat sink is 26°C, 5°C. For the variation of temperature difference between the heat source and heat sink, when the temperature difference was 21°C, the gross power increased by about 33.3% from that when the temperature difference was 19 °C.

The present study uses a heat sink plate to conduct natural convection in order to examine different areas of the heat sink and the effects of mounting different quantities of LEDs on the same surface on the thermal mechanism performance. Based on the experimental results, when a heat sink plate is arranged vertically, the channel flow between the fins is parallel to gravity. The LED substrate plate temperature is different from that at the end of the fin, and rises with the increase of total power. The thermal resistance rises slowly and then declines with the increase of LED electric power. As for temperature change of the LED substrate and at the end of the fin, when the temperature difference is increased, it also increases the natural convection thrust. For thermal resistance, the environmental thermal resistance at the bottom of the heat sink plate is lower than at the middle and top sections. These LED power emissions will be changed synchronously.Regarding the LED quantity control, the rate of increase is the highest for the heat sink plate with 30 pcs LED, and the temperature is very high for the heat sink plate with 45 and 60 pcs LEDs when the power approaches 1 W. Moreover, the rising rate is the lowest for the heat sink plate with 60 pcs LEDs. Depending on the brightness requirement, the illuminant is provided by 60 pcs LEDs to obtain a lower temperature so that the system can reduce the thermal protective design. Evidence shows that a high conductivity heat pipe embedded in the channel can provide a more uniform temperature distribution. The present study provides a further understanding on the influence of different illuminant densities on the heat sink structure and the temperature difference in an LED heat transfer device, in order to provide a reference for heat sink design of a backlight module and LED illuminant module evaluation. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/htj.20321

Wearable thermoelectric generators have great potential to provide power for smart electronic wearable devices and miniature sensors by harnessing the temperature difference between the human body and the environment. However, the Thomson effect, the Joule effect, and heat conduction can cause a decrease in the temperature difference across the thermoelectric generator during operation. In this paper, phase change materials (PCMs) were employed as the heat sink for the thermoelectric generator, and the COMSOL software 6.1 was utilized to simulate and optimize the power generation processes within the heat sink. The results indicated that with a PCM height of 40 mm, phase transition temperature of 293 K, latent heat of 200 kJ/kg, phase transition temperature interval of 5 K, thermal conductivity of 50 W/(m·K), isobaric heat capacity of 2000 J/(Kg·K), density of 1000 kg/m3, and convective heat transfer coefficient of 10 W/(m·K), the device can maintain a temperature difference of 18-10 K for 1930 s when the thermoelectric leg height is 1.6 mm, and 3760 s when the thermoelectric leg height is 2.7 mm. These results demonstrate the correlation between the device's output performance and the dimensions and performance parameters of the PCM heat sink, thereby validating the feasibility of employing the PCM heat sink and the necessity for systematic investigations.