Innovative Volumetric Solar Receiver Micro-Design Based on Numerical Predictions

Due to the continuous global increase in energy demand, Concentrated Solar Power (CSP) represents an excellent alternative, or add-on to existing systems for the production of energy on a large scale. In some of these systems, the Solar Power Tower plants (SPT), the conversion of solar radiation into heat occurs in certain components defined as solar receivers, placed in correspondence of the focus of the reflected sunlight. In a particular type of solar receivers, defined as volumetric, the use of porous materials is foreseen. These receivers are characterized by a porous structure called absorber. The latter, hit by the reflected solar radiation, transfers the heat to the evolving fluid, generally air subject to natural convection. The proper design of these elements is essential in order to achieve high efficiencies, making such structures extremely beneficial for the overall performances of the energy production process. In the following study, a parametric analysis and an optimized characterization of the structure have been performed with the use of self-developed numerical models. The knowledge and results gained through this study have been used to define an optimization path in order to improve the absorber microstructure, starting from the current in-house state-of-the-art technology until obtaining a new advanced geometry.

Study on Periodic Thermal-Switching Behavior of Flat Heat Pipe

The coupling of the electrocaloric effect in thin films with thermal switches has the potential to be used for efficient refrigeration. We studied the unsteady heat transfer performance and periodic thermal-switching behavior of a flat heat pipe to transfer cold energy from a changing heat source. The condenser of the flat heat pipe was the changing heat source and changed from −20 W to +20 W every 5 s. The temperature of the condenser surface changed in accordance with the heat generation of the heat source. The evaporator was a plate with a mesh wick attached to a water-flow pipe. Cold energy transferred from the condenser surface to the evaporator surface only when the temperature of the condenser surface was lower than that of the evaporator surface. We analyzed the unsteady temperature change and heat transfer performance of the flat heat pipe by numerical simulation. The analytical results showed that it was necessary to have two thermal switches to separate the heat energy and cold energy of the changing heat source. Also, it was important to reduce the thermal resistance and heat capacity of the evaporator surface to improve the unsteady heat transfer performance of the heat pipe. Next, we measured the unsteady heat transfer performance of the flat heat pipe experimentally. The experimental results showed that the thermal-switching behavior was observed when the heat generation of the heat source changed every 5 s.

Enhancement of Heat Transfer Performance in Nuclear Fuel Rod Using Nanofluids and Surface Roughness Technique

The experimental investigations were carried out of a pressurized water nuclear reactor (PWR) with enhanced surface using different concentration (0.5 and 2.0 vol%) of ZnO/DI-water based nanofluids as a coolant. The experimental setup consisted of a flow loop with a nuclear fuel rod section that was heated by electrical current. The fuel rod surfaces were termed as two-dimensional surface roughness (square transverse ribbed surface) and three-dimensional surface roughness (diamond shaped blocks). The variation in temperature of nuclear fuel rod was measured along the length of a specified section. Heat transfer coefficient was calculated by measuring heat flux and temperature differences between surface and bulk fluid. The experimental results of nanofluids were compared with the coolant as a DI-water data. The maximum heat transfer coefficient enhancement was achieved 33% at Re = 1.15 × 105 for fuel rod with three-dimensional surface roughness using 2.0 vol% nanofluids compared to DI-water.

Perimeter Cooling Unit and Localized Row-Based Cooling Unit Transient Air Flow Effects Modeling and Characterization in Data Centers

Cooling power constitutes a large portion of the total electrical power consumption in data centers. Approximately 25%∼40% of the electricity used within a production data center is consumed by the cooling system. Improving the cooling energy efficiency has attracted a great deal of research attention. Many strategies have been proposed for cutting the data center energy costs. One of the effective strategies for increasing the cooling efficiency is using dynamic thermal management. Another effective strategy is placing cooling devices (heat exchangers) closer to the source of heat. This is the basic design principle of many hybrid cooling systems and liquid cooling systems for data centers. Dynamic thermal management of data centers is a huge challenge, due to the fact that data centers are operated under complex dynamic conditions, even during normal operating conditions. In addition, hybrid cooling systems for data centers introduce additional localized cooling devices, such as in row cooling units and overhead coolers, which significantly increase the complexity of dynamic thermal management. Therefore, it is of paramount importance to characterize the dynamic responses of data centers under variations from different cooling units, such as cooling air flow rate variations. In this study, a detailed computational analysis of an in row cooler based hybrid cooled data center is conducted using a commercially available computational fluid dynamics (CFD) code. A representative CFD model for a raised floor data center with cold aisle-hot aisle arrangement fashion is developed. The hybrid cooling system is designed using perimeter CRAH units and localized in row cooling units. The CRAH unit supplies centralized cooling air to the under floor plenum, and the cooling air enters the cold aisle through perforated tiles. The in row cooling unit is located on the raised floor between the server racks. It supplies the cooling air directly to the cold aisle, and intakes hot air from the back of the racks (hot aisle). Therefore, two different cooling air sources are supplied to the cold aisle, but the ways they are delivered to the cold aisle are different. Several modeling cases are designed to study the transient effects of variations in the flow rates of the two cooling air sources. The server power and the cooling air flow variation combination scenarios are also modeled and studied. The detailed impacts of each modeling case on the rack inlet air temperature and cold aisle air flow distribution are studied. The results presented in this work provide an understanding of the effects of air flow variations on the thermal performance of data centers. The results and corresponding analysis is used for improving the running efficiency of this type of raised floor hybrid data centers using CRAH and IRC units.

Study of the Fluid Dynamics of Thin Liquid Films Flowing Down a Vertical String With Counterflow of Gas

Direct-contact heat transfer between a falling liquid film and a gas stream yield high heat transfer rates and as such it is routinely used in several industrial applications. This concept has been incorporated by us into the proposed design of a novel heat exchanger for indirect cooling of steam in power plants. The DILSHE (Direct-contact Liquid-on-String Heat Exchangers) module consists of an array of small diameter (∼ 1 mm) vertical strings with hot liquid coolant flowing down them due to gravity. A low- or near-zero vapor pressure liquid coolant is essential to minimize/eliminate coolant loss. Consequently, liquids such as Ionic Liquids and Silicone oils are ideal candidates for the coolant. The liquid film thickness is of the order of 1 mm. Gas (ambient air) flowing upwards cools the hot liquid coolant. Onset of fluid instabilities (Rayleigh-Plateau and/or Kapitza instabilities) result in the formation of a liquid beads, which enhance heat transfer due to additional mixing. The key to successfully designing and operating DILSHE is understanding the fundamentals of the liquid film fluid dynamics and heat transfer and developing an operational performance map. As a first step towards achieving these goals, we have undertaken a parametric experimental and numerical study to investigate the fluid dynamics of thin liquid films flowing down small diameter strings. Silicone oil and air are the working fluids in the experiments. The experiments were performed with a single nylon sting (fishing line) of diameter = 0.61 mm and height = 1.6 m. The inlet temperature of both liquid and air were constant (∼ 20 °C). In the present set of experiments the variables that were parametrically varied were: (i) liquid mass flow rate (0.05 to 0.23 g/s) and (ii) average air velocity (0 to 2.7 m/s). Visualization of the liquid flow was performed using a high-speed camera. Parameters such as base liquid film thickness, liquid bead shape and size, velocity (and hence frequency) of beads were measured from the high-speed video recordings. The effect of gas velocity on the dynamics of the liquid beads was compared to data available in the open literature. Within the range of gas velocities used in the experiments, the occurrence of liquid hold up and/or liquid blow over, if any, were also identified. Numerical simulations of the two-phase flow are currently being performed. The experimental results will be invaluable in validation/refinement of the numerical simulations and development of the operational map.

An Experimental Study of the Effect of SiO2 Nanoparticles on the Phase Change Characteristics of KNO3-NaNO3 Mixtures for Thermal Energy Storage

Combining latent and sensible heat storage within a single material has lead researchers to propose off-eutectic salt mixtures for solar thermal energy storage. A binary salt mixture with an off-eutectic composition presents a melting temperature range as opposed to a melting point, providing additional storage capacity through the sensible heat present during the melting/solidification. However, some drawbacks of off-eutectic mixtures include a higher propensity towards incongruous melting, phase segregation of the mixture upon solidification, and thermal cycling issues. Here the solid-liquid transitions of a series of eutectic and off-eutectic KNO3-NaNO3 mixtures are evaluated to determine if the addition of nanoparticles can limit phase segregation yet still present good thermal properties. Measurements performed with a Differential Scanning Calorimeter show that the effect of nanoparticles is unimportant compared to the difference between eutectic and off-eutectic phase change behavior. Onset temperatures, latent heats, and the width of the phase transition temperature trace during melting/solidification are compared for the eutectic and off-eutectic salt mixtures with and without silica nanoparticles. A reduction in the latent heat is observed and explained through classical mixing theory. The modification of the phase change properties of eutectic and off-eutectic nanofluids are discussed.

Steel Structure Prediction for Continuous Steel Casting by Means of a Parallel GPU-Based Heat Transfer and Solidification Model

Continuous casting of steel is currently a predominant production method of steel, which is used for more than 95% of the total world steel production. An effort of steelmakers is to cast high-quality steel with a desired structure and with a minimum number of defects, which reduce the productivity. The paper presents our developed GPU-based heat transfer and solidification model for continuous casting, which is coupled with a submodel used for the prediction of the steel micro-structure. The model is implemented in CUDA/C++, which allows for rapid computing on NVIDIA GPUs. The time-dependent temperature distribution calculated by the thermal model is iteratively passed to the submodel for the steel micro-structure prediction. The structural submodel determines the spatially-dependent rates of temperature change in the strand, for which the interdendritic solidification model IDS predicts the micro-structure of steel. The paper presents preliminary simulation results for the steel grade used for pressure vessel plates, which is sensitive to rapid cooling rates.

A Ventilation Cooling Solution to Improve Thermal Environment of High Energy Density Li-Ion Battery Packs

This paper focuses on the cooling solution to a high energy density and large capacity Li-ion battery system which consist of four packs of 26650 cells. The cooling measure is a critical technology for many Li-ion battery systems especially that designed for hybrid electric vehicles, in which, high energy density within a limited space is very common in these systems. Both the safety and efficiency of Li-ion battery cells rely on the temperature which is under control of the battery thermal management system. In this study, temperature fields within battery boxes are simulated with the computational fluid dynamic (CFD) method. With the help of an airconditioner, a cooling solution is proposed for a relatively large dimensional, high energy density Li-ion battery cells array using by vehicles. Through the proposed solution, the maximum single-cell temperature is restricted to a reasonable level, and the maximum temperature difference throughout the battery system is also improved.

Spray Cooling With HFC-134a and HFO-1234yf for Thermal Management of Automotive Power Electronics

This study aimed to experimentally investigate the spray cooling characteristics for active two-phase cooling of automotive power electronics. Tests were conducted on a small-scale, closed loop spray cooling system featuring a pressure atomized spray nozzle. Two types of refrigerants, HFC-134a (R-134a) and HFO-1234yf, were selected as the working fluids. The test section (heater), made out of oxygen-free copper, had a 1-cm2 plain, smooth surface prepared following a consistent procedure, and served as a baseline case. Matching size thick film resistors, attached onto the copper heaters, generated heat and simulated high heat flux power electronics devices. The experiments were performed with saturated working fluids at room temperature level (22°C) by controlling the heat flux in increasing steps, and recording the corresponding steady-state temperatures to obtain cooling curves. Performance comparisons were made based on heat transfer coefficient (HTC) and critical heat flux (CHF) values. Effects of spray characteristics and liquid flow rates on the cooling performance were determined with three types of commercially available nozzles that generate full-cone sprays with fine droplets, and with varying flow rates between 1.6 to 5.4 ml/cm2.s. The experimental data showed that HFC-134a provided better performance compared to HFO-1234yf, in terms of HTC and CHF, which is believed to be dictated by the thermophysical properties that affect both the spray characteristics and heat acquisition ability. Overall, this study provided a framework for spray cooling performance with the current and next-generation refrigerants aimed for advanced thermal management of automotive power electronics.