Turbulent Natural Convection in a Differentially Heated Vertical Channel

The turbulence natural convection boundary layer inside a infinite vertical channel with differentially heated walls is analyzed based on a similarity solution methodology. The differences between mean temperature and velocity profiles in a boundary layer along a vertical flat plate and in a channel flow, make it necessary to introduce new sets of scaling parameters. In the limit as H* → ∞, two distinctive parts are considered: an outer region which dominates the core of the flow and inner constant heat flux region close to the walls. The proper inner scaling velocity is showed to be determined by the outer parameters due to momentum integral. The theory is contrasted with the one suggested by George & Capp (1), the deficiencies of which are identified.

The Effect of Partitions on the Laminar Natural Convection in a Square Cavity

The effect of a partition on the laminar natural convection flow in an air-filled square cavity driven by a temperature difference across the vertical walls was investigated experimentally. Two partitions with non-dimensional heights of 0.0625 and 0.125 was attached either to the upper half of the heated vertical wall or the top wall at different locations. The experiments were performed for a global Grashof number of approximately 1.24×108 and non-dimensional top wall temperatures of approximately 0.48 to 2.28. At the higher top wall temperatures, a secondary flow circulation region formed between the partition attached to the top wall and the heated vertical wall of the cavity. This secondary flow circulation region was sensitive to the location and height of the partition, in addition to the top wall temperature of the cavity. The secondary flow circulation region moved the location where the upward boundary layer flow along the heated vertical wall turned over to be further away from the top wall, than in the cavity without the partition. A thermal boundary layer was observed to move along the rear surface of the partition attached to the top wall. In the region close to the top wall, the partitions caused the non-dimensional temperature outside of the boundary layer and the local Nusselt number along the heated vertical wall to be different from that in the cavity without the partition. There were no significant effects of the partition on the flow and heat transfer characteristics in the lower half of the cavity.

An Analysis of a Direct Contact Ice Slurry Generator

Attempts have been made to study an ice slurry generation system where two immiscible liquids, water and a coolant, are used to produce ice slurry by direct contact heat transfer. A mathematical model has been developed to evaluate the heat transfer phenomena between the coolant drops and the water in the ice slurry generation system. In this process, all the important variables that affect the direct contact heat transfer between these two fluids were incorporated into the simulation model to evaluate thermal performance of the system. Experiments were performed on an ice slurry generator using water and an immiscible liquid coolant, Fluroinert FC-84. The coolant at about −10°C to −15°C was delivered to the top of the ice slurry generator containing water and collected from the bottom for recirculation. The measured temperature profiles of water in the ice slurry generator for different coolant flow rates (8 lit/min to 12 lit/min) showed a good agreement with those temperature profiles obtained from the simulation model. These results validated the simulation model developed for the ice slurry generator. The analysis showed that during sensible cooling, the estimated heat transfer coefficients between water and the coolant were in the range of 3.0 to 6.5 kW/m2 for coolant flow rates varying from 8 lit/min to 12 lit/min. Higher coolant flow rates also enhanced the ice formation process due to the increased heat transfer rate. In addition, it was also observed that the ice production increased significantly when the nozzle was placed at the bottom of the ice slurry generator.

Designing Si/Si1−xGex Superlattices With Tailored Thermal Transport Properties

Si/Si1−xGex superlattices are promising candidates for thermoelectric energy conversion applications [1, 2], as the phonon transport through them can be inhibited while maintaining desirable electrical transport properties. No comprehensive experimental study has been performed to map the thermal conductivity design space accessible by Si/Ge nanocomposites. By using atomistic modeling tools, interesting areas of the design space can be identified and then further explored experimentally.

Reverse Monte-Carlo Ray-Tracing Method Combined With Full-Spectrum K-Distribution Method for Radiative Heat Transfer

Accurate prediction of radiative heat transfer plays a key role in many high temperature applications, such as combustion devices and fires. Among various simulation methods, the Monte-Carlo Ray-Tracing (MCRT) has the advantage of solving the radiative transfer equation (RTE) for real gas mixtures with almost no approximations; however, it has disadvantage of requiring a large computational effort. The MCRT method can be carried out with either the Forward MCRT or the Reverse MCRT, depending on the direction of ray tracing. The RMCRT method has advantages over the FMCRT method in that it uses less memory, and in a domain decomposition parallelization strategy, it can explicitly obtain solutions for the domain of interest without the need for the solution on the entire domain.

Effects of Channel Height and Bulk Temperature Considerations on Heat Transfer Coefficient of Wetted Surfaces in a Single Inline Row Impingement Channel

High performance turbine airfoils are typically cooled with a combination of internal cooling channels and impingement/film cooling. In such applications, the jets impinge against a target surface, and then exit along the channel formed by the jet plate, target plate, and side walls. Local convection coefficients are the result of both the jet impact, as well as the channel flow produced from the exiting jets. Numerous studies have explored the effects of jet array and channel configurations on both target and jet plate heat transfer coefficients. However, little work has been done in examining effects on the channel side walls, which may be a major contributor to heat transfer in real world applications. This paper examines the local and averaged effects of channel height and on heat transfer coefficients, with special attention given to the channel side walls. The effects on heat transfer results due to bulk temperature variations were also investigated. High resolution local heat transfer coefficient distributions on target and side wall surfaces were measured using temperature sensitive paint and recorded via a scientific grade charge-coupled device (CCD) camera. Streamwise pressure distributions for both the target and side walls was recorded and used to explain heat transfer trends. Results are presented for average jet based Reynolds numbers between 17,000 and 45,000. All experiments were carried out on a large scale single row, 15 hole impingement channel, with X/D of 5, Y/D of 4, and Z/D of 1, 3 and 5. The results obtained from this investigation will aid in the validation of predictive tools and development of physics-based models.

Experimental Study on Double-Diffusive Convection During Solidification of NH4Cl-H2O Hypereutectic Solution in Cylinder Cavity

In order to reveal the law of double-diffusive convection of multi-compound solution in cylindrical cavity, experimental study on solidification of NH4Cl-H2O hypereutectic solution has been performed by using particle image velocimetry (PIV). The influencing factors of flow patterns and intensity are also analyzed. The results show that: 1) There are two approximately symmetric main convection cells in the liquid which are down along the sidewall and up along the center of the cylindrical cavity. Meanwhile, there are also two secondary cells on the bottom corner of cylindrical cavity, which flow in contrary direction to that of the main ones; 2) Due to the release of water during the solidification process, solute layers and diffusive interface are developed in the liquid and will be disappeared in the end; 3) The cooling temperature and the initial concentration have significantly effects on the flow velocity, solute layers and diffusive interface.

Numerical Simulation of Laminar Confined Impinging Jet in a Composite Channel

This work shows numerical results for a jet impinging onto a flat plane covered with a layer of a porous material. Porosity of the porous layer is varied in order to analyze its effect on the local distribution of Nu. Macroscopic equations for mass and momentum ae obtained based on the volume-average concept. The numerical technique employed for discretizing the governing equations was the control volume method with a boundary-fitted non-orthogonal coordinate system. The SIMPLE algorithm was used to handle the pressure-velocity coupling. Results indicate that inclusion of a porous layer decreases the peak in Nu avoiding excessive heating or cooling near the stagnation region.

Temperature and Concentration Simulations in the Methanol Steam Reformer

The methanol steam reforming plays an important role for hydrogen supply to the proton membrane exchange fuel cell in the portable power applications. The catalyst coating on the walls of channels is often used in the fabrication of the reactors in the reformer to minimize the pressure loss. In this study, the temperature and concentration fields in the reactors for the methanol steam reforming were investigated numerically. The methanol conversion is usually used to evaluate the performance of the reformer. The effects of the inlet gas temperature in the heat supply channel and inlet velocity in the reforming channel on the performance of the methanol steam reforming are presented.

Performance of Thermal Enhancement Materials in High Pressure Metal Hydride Storage Systems

Over the past two years, key issues associated with the development of realistic metal hydride storage systems have been identified and studied at Purdue University’s Hydrogen Systems Laboratory, part of the Energy Center at Discovery Park. Ongoing research projects are aimed at the demonstration of a prototype large-scale metal hydride tank that achieves fill and release rates compatible with current automotive use. The large-scale storage system is a prototype with multiple pressure vessels compatible with 350 bar operation. Tests are conducted at the Hydrogen Systems Lab in a 1000 ft2 laboratory space comprised of two test cells and a control room that has been upgraded for hydrogen service compatibility. The infrastructure and associated data acquisition and control systems allow for remote testing with several kilograms of high-pressure reversible metal hydride powder. Managing the large amount of heat generated during hydrogen loading directly affects the refueling time. However, the thermal management of hydride systems is problematic because of the low thermal conductivity of the metal hydrides (∼ 1 W/m-K). Current efforts are aimed at optimizing the filling-dependent thermal performance of the metal hydride storage system to minimize the refueling time of a practical system. Combined heat conduction within the metal hydride and the enhancing material particles, across the contacts of particles and within the hydrogen gas between non-contacted particles plays a critical role in dissipating heat to sustain high reaction rates during refueling. Methods to increase the effective thermal conductivity of metal hydride powders include using additives with substantially higher thermal conductivity such as aluminum, graphite, metal foams and carbon nanotubes. This paper presents the results of experimental studies in which various thermal enhancement materials are added to the metal hydride powder in an effort to maximize the effective thermal conductivity of the test bed. The size, aspect ratio, and intrinsic thermal conductivity of the enhancement materials are taken into account to adapt heat conduction models through composite nanoporous media. Thermal conductivity and density of the composite materials are measured and enhancement metrics are calculated to rate performance of composites. Experimental results of the hydriding process of thermally enhanced metal hydride powder are compared to un-enhanced metal hydride powder and to model predictions. The development of the Hydrogen Systems Laboratory is also discussed in light of the lessons learned in managing large quantities of metal hydride and high pressure hydrogen gas.