Fully Coupled Elliptic Numerical Model for Film Condensation From Vapour-Gas Mixtures in Vertical Parallel Plate Channels

Laminar film condensation is an important phenomenon which occurs in numerous industrial applications such as refrigeration, chemical processing, and thermal power generation industries. It is well known that film condensation heat transfer is greatly reduced in the presence of a non-condensing gas. The present work performs a numerical analysis of the steady-state, laminar film condensation from a vapour-gas mixture in vertical parallel plate channels to demonstrate a computer model that could assist engineering analysts designing systems involving these phenomena. The present model has three new aspects relative to other current work. First, the complete elliptic two-dimensional governing equations are solved in both phases. Thus, the entire channel domain is solved rather than using an approach that marches along the channel from inlet to a prescribed length. Second, a dynamically determined sharp interface is used between the phases. This sharp interface is determined during the solution on a non-orthogonal structured mesh. Third, the governing equations are solved in a fully-coupled approach. The equations for two velocities, pressure, temperature, and gas mass fraction are solved in a coupled method simultaneously for both phases. Discretisation has been done based on a finite volume method and a co-located variable storage scheme. An in-house computer code was developed to implement the numerical solution scheme. Detailed results are presented for laminar film condensation from steam-air mixtures flowing in vertical parallel-plate channels. The results include velocity and pressure profiles, as well as axial variations of film thickness, Nusselt number and interface gas mass fraction. Detailed comparisons are made with results from a parabolic solution approach.

Vortex-Induced Vibration for Heat Transfer Enhancement

A passive, self-agitating method which takes advantage of vortex-induced vibration (VIV) is presented to disrupt the thermal boundary layer and thereby enhance the convective heat transfer performance of a channel. A flexible cylinder is placed at centerline of a channel. The vortex shedding due to the presence of the cylinder generates a periodic lift force and the consequent vibration of the cylinder. The fluid-structure-interaction (FSI) due to the vibration strengthens the disruption of the thermal boundary layer by reinforcing vortex interaction with the walls, and improves the mixing process. This novel concept is demonstrated by a three-dimensional modeling study in different channels. The fluid dynamics and thermal performance are discussed in terms of the vortex dynamics, disruption of the thermal boundary layer, local and average Nusselt numbers (Nu), and pressure loss. At different conditions (Reynolds numbers, channel geometries, material properties), the channel with the VIV is seen to significantly increase the convective heat transfer coefficient. When the Reynolds number is 168, the channel with the VIV improves the average Nu by 234.8% and 51.4% in comparison with a clean channel and a channel with a stationary cylinder, respectively. The cylinder with the natural frequency close to the vortex shedding frequency is proved to have the maximum heat transfer enhancement. When the natural frequency is different from the vortex shedding frequency, the lower natural frequency shows a higher heat transfer rate and lower pressure loss than the larger one.

Non-Surface Polaritonic Peaks in Near-Field Radiative Transfer

Near-field effects in radiative transfer refer to the collective influence of interference, diffraction, and tunneling of electro-magnetic waves on energy transfer between two or more objects. Most studies of near-field radiative transfer have so far focused on the enhancement due to tunneling of surface polaritons. In this work, we show the existence of sharp peaks in the radiative transfer spectrum between two spheres of polar materials that are not due to surface polaritons. The peaks, which are present on either side of the restrahlen band, are because of Mie resonances.

An Investigation of Methane Combustion in a Rectangular Shaped Meso Chamber

Hydrocarbon-based miniature power generators are promising any many application since hydrocarbon based fuels have higher power densities compared to conventional lithium batteries. A 40mm long meso-combustor of two different configurations, two-inlet and three-inlet, were used to investigate the combustion of methane in the meso-chamber. A non-premixed combustion of methane and oxygen was simulated numerically using a steady laminar flamelet model. The mesh generation and the CFD simulation were performed using ANSYS FLUENT software. A a finite volume approach was used for the simulation. The fuel-oxidizer mixing, thermal behavior and fuel burning efficiency were studied. An adequate mixing that supports the combustion was observed in certain locations. The exhaust gas was analyzed experimentally. The temperature distributions were also observed to predict the flame locations. According to the numerical analysis it was apparent that the flame would be anchored in the well mixed regions of the chamber the flames were found to be attached in two distinct locations. One in the upstream zone and the other one in the downstream zone. Another important finding was that the fuel lean condition produced higher efficiency than the fuel rich condition.

Effects of Water Injection on the Power Boost of a Twin Turbocharged Vehicle

The injection of a water/methanol mixture into the intake airflow of turbocharged street racing vehicles has long been known to increase the power and performance, though actual data proves difficult to obtain. In an attempt to maximize the power output of a twin-turbocharged Porsche engine, a water/methanol mixture is introduced into the intake airflow. The injection decreased the temperature and increased the density of the intake air while increasing the compression ratio of the cylinder beyond the effects of the turbocharger itself. The real-time data was used to do the analysis. In an attempt to maximize the power enhancement, the injection parameters are modified. The vehicle sensors were used to obtain temperatures of the intake air, exhaust air, and the engine coolant. The fuel flow rate, air flow rate, mixture flow rate, and manifold pressure in conjunction with temperature measurements was used to determine the mass flow rate of the system. The properties of the mixture components were used to determine the work created by the system, which in turn leads to the power output of the system. Determining the power enhancement created by the water/methanol injection, and potentially maximizing its effect is the focus of this work.

CFD Study of a Fire Whirl of Huge Oil Tank: Burning Rate, Flame Length, Distributions of n-Heptane and Oxygen in a Fire Whirl

The number of huge oil storage tanks is increasing in the world. If a fire occurs in one of these tanks, it is very difficult to suppress. Additionally, if a fire whirl occurs in an oil tank fire, it is extremely dangerous for firefighters to extinguish the fire. The authors have numerically studied huge fire whirls in a large oil tank depot and predicted the generation of those fire whirls. Here, another study is attempted to clarify the details of huge fire whirl in a large oil tank, using two kinds of fire whirl generation channels in CFD simulations using the software, FDS by NIST. Details of burning rates, velocities of whirling flames, radiative heat flux, heat release rates and whirling cycles are examined, using oil tanks with the diameters of 0.2 to 80 m. In oil tanks with a diameter of 80 m, a tall fire whirl is generated. The height is about 1000 m. In this study of oil tanks fires with small to large diameters, it has been found that fire whirl lengths are about 8 to 11 times of the oil tank diameter. The maximum radiative heat flux due to a fire whirl in 80 m diameter oil tanks exceeds 100 kW/m2. Since the maximum radiation is found at twice the distance of oil tank diameters from the tank centers, adjacent oil tanks may be ignited. This study has also examined a method used to prevent fire whirl generation in huge oil tanks.

Enhanced Passive Thermal Management of Grid-Scale Power Routers Utilizing Ionic Wind

This paper presents a numerical model assessing the potential of ionic wind as a heat transfer enhancement method for the cooling of grid distribution assets. Distribution scale power routers (13–37 kV, 1–10 MW) have stringent requirements regarding lifetime and reliability, so that any cooling technique involving moving parts such as fans or pumps are not viable. Increasing the air flow — and thereby enhancing heat transfer — through Corona discharge could be an attractive solution to the thermal design of such devices. In this work, the geometry of a rectangular, vertical channel with a corona electrode at the entrance is considered. The multiphysics problem is characterized by a set of four differential equations: the Poisson equation for the electric field and conservation equations for electric charges, momentum and energy. The electrodynamics part of the problem is solved using a finite difference approximation (FDA). Solutions for the potential, electric field and free charge density are presented for a rectangular control volume with mixed boundary conditions.

Development of a Multi-Loop Flow and Heat Transfer Facility for Advanced Nuclear Reactor Thermal Hydraulic and Hybrid Energy System Studies

A new high-temperature multi-fluid, multi-loop test facility for advanced nuclear applications is under development at the Idaho National Laboratory. The facility will include three flow loops: high-temperature helium, molten salt, and steam/water. Molten salts have been identified as excellent candidate heat transport fluids for primary or secondary coolant loops, supporting advanced high temperature and small modular reactors (SMRs). Details of some of the design aspects and challenges of this facility, which is currently in the conceptual design phase, are discussed. A preliminary design configuration will be presented, with the required characteristics of the various components. The loop will utilize advanced high-temperature compact printed-circuit heat exchangers (PCHEs) operating at prototypic intermediate heat exchanger (IHX) conditions. The initial configuration will include a high-temperature (750°C), high-pressure (7 MPa) helium loop thermally integrated with a molten fluoride salt (KF-ZrF4) flow loop operating at low pressure (0.2 MPa) at a temperature of ∼450°C. Experiment design challenges include identification of suitable materials and components that will withstand the required loop operating conditions. Corrosion and high temperature creep behavior are major considerations. The facility will include a thermal energy storage capability designed to support scaled process heat delivery for a variety of hybrid energy systems and grid stabilization strategies. Experimental results obtained from this research will also provide important data for code verification and validation (V&V) related to these systems.

Investigation of Building Passive Thermal Storage for Optimal Heating System Design

Heating and cooling load calculations are critical to size Heating, Ventilation and Air conditioning (HVAC) systems and determine energy use of their operations. The ASHRAE (2009) model, which is most commonly used for heating load calculations, adopts a simplified approach by considering only steady-state instantaneous conductive heat transfer and ignoring internal heat gains and thermal storage effects. Those assumptions evaluate the worst case conditions which can reasonably occur at nights when the outdoor air temperature is lowest and with no inputs from solar, occupants, lights, or any electronic devices. However, due to thermal storage effect, heat generated in daytime can be still stored in buildings. Such ignorance leads to significantly over-sized heating system, high initial cost and a higher cost of energy uses. On the other hand, by considering passive thermal storage of buildings and allowing space air to drift to reasonably lower values, buildings need to be warmed up in the morning before being occupied. The worst case conditions might happen in the morning warm-up period, when heating is needed. This study therefore examines the thermal response of different constructions (heavy, medium, and light) of the building envelope and investigates the effect of their passive thermal storage on the size of the heating system. Results show tremendous opportunities for downsizing of the heating system while still maintaining thermal comfort requirements. As such, this paper is a fundamental study of building thermal characteristics in order to investigate the potentials of establishing a new heating device design standard.

Dryout Avoidance Control for Multi-Evaporator Vapor Compression Cycles With Transient Heat Flux

Multiple-evaporator vapor compression cycles may be used for distributed cooling of high heat-flux systems, such as arrays of high-power electronics. Under transient heating conditions, these systems must be carefully controlled to avoid critical heat flux (CHF) due to evaporator dryout. An active control strategy is presented that regulates two-phase flow quality in multiple evaporators in order to avoid critical quality under transient heating conditions. A two-loop control system is used, in which an outer loop uses model-based feedforward combined with evaporator wall temperature feedback to determine the necessary coolant flow rate to avoid CHF, while an inner loop uses system actuators (variable speed compressor, electronic expansion valves) to track to the desired flow rate. An advantage of this approach is that the inner-loop control handles the system complexity arising from pressure coupling and actuator nonlinearity. Additionally, the outer-loop quality control may be applied to other two-phase cooling schemes, for instance pumped systems, by providing coolant flow rate setpoints. Simulations and corresponding experimental controller validation were conducted using a three-evaporator vapor compression testbed with transient imposed heat-flux.