J.S. Cotton

Characterization of the Soot Deposition Profiles in Diesel Engine Exhaust Gas Recirculation (EGR) Cooling Devices Using a Digital Neutron Radiography Imaging Technique

A non-destructive neutron radiography technique was used to measure the thickness of diesel soot deposited in the tubes of exhaust gas recirculation (EGR) cooling devices. Measurements were performed to characterize the fouling in single-tube and three-tube devices for laminar and turbulent flows. Measurements were also performed to characterize the effect that the design of the inlet header had on the deposition characteristics in the device. The analysis of the neutron images showed that the soot deposition in the single-tube device occurred at a faster rate for a turbulent flow than for a laminar flow. The deposition thickness decreased along the tubes for both flow regimes. More soot deposited in the center tube of the three-tube bundle for the expansion angle 45° inlet header suggesting there was an uneven distribution of the exhaust gas flow in the tube bundle. For the device with the expansion angle 60° inlet header the soot was approximately more evenly distributed along the tubes.

Numerical simulation of electric field distributions in electrohydrodynamic two-phase flow regimes

In electrohydrodynamic (EHD) flow boiling and condensation heat transfer applications an interdependence exists between two-phase flow patterns and the applied voltage, and subsequently the electric field distribution established. Unlike single-phase flow, in liquid/gas or phase change processes the electric field established is continuously changing as the flow pattern changes due to interfacial vaporization or condensation. To further complicate the variation in this dynamic field effect, the interaction between the electric field and the fluid introduce an electrical force that can also cause a redistribution of the phases. In an effort to understand and analyze this interaction, the electric field distribution must be determined. To contribute to this effort, the evaluation of the static electric field distribution is performed for various flow regimes to provide a qualitative assessment regarding the direction of phase migration and possible flow pattern transition and,to determine the net EHD force acting on the flow for an instant in time, i.e. for a given phase distribution.

Analysis of quasi-static vapour bubble shape during growth and departure

In an effort to better understand the physical mechanisms responsible for pool boiling heat transfer, a numerical solution to the capillary equation is used to describe bubble shape evolution. Indeed, any analysis of thermal transport due to nucleate pool boiling requires bubble frequency and volume predictions, which are intimately linked to bubble shape. To this end, a numerical treatment of the capillary equation is benchmarked to profiles measured from captured images of vapour bubble formations. The bubble growth is quasi-static in a quiescent liquid with a triple contact line fixed to the perimeter of a needle orifice. This investigation provides insight into the dependence the bubble shape evolution has on the physical mechanisms quantified in the Bond number with characteristic length equal to the cavity radius.

Evaluation of thermal energy storage and recovery for an electrical energy mediator system

Abstract Energy storage both electrical and thermal is a rapidly emerging field of interest toward the development of more sustainable energy systems. The inherent inefficiencies associate with electrical storage can be partially overcome when thermal storage that collects and storage the waste thermal energy for alternative uses is integrated. Consequently, thermal energy storage systems are an enabling technology that will allow increased energy efficiency of a community, permit load levelling to reduce peak electricity demand. In order to facilitate a technology evaluation, a sizing strategy is developed for a phase change material (PCM) thermal storage system that determines system requirements under given thermal energy capture and recovery cycles. The sizing process utilizes a simplified one-dimensional heat transfer model that estimates melt times for a phase change material thickness without detailed geometry information. This melt time estimate allows the proportion of phase change material to fluid routing materials to be calculated, giving an estimate of material cost for the thermal storage cell to determine economic feasibility. The model is compared to both experimental data and computational fluid dynamics models in order to determine its limitations. Through a specific example of hydrogen based distributed electrical energy mediator system, the utility of the sizing model in determining the estimated cost of thermal energy storage is demonstrated.

Visualization of Flow Regime Transitions in Two-Phase Flow Under High Voltage Electric Fields

The effects of applying DC high voltage electric fields on two-phase flow regime transitions for flowing refrigerant HFC-134a were visualized using a high speed camera. The viewing test section was made of 10 mm inner diameter quartz tube with a 3.18 mm diameter charged electrode placed along the center of the tube. The quartz tube was coated with an electrically grounded transparent conductive film of Tin Oxide. The experiments were performed for mass flux (55 kg/m2 s < G < 263 kg/m2 s), quality (20% < x < 80%) and applied voltage (0 kV < V < 8 kV). The flow regime transitions depend on the flow regime prior to applying the EHD. For stratified flow, EHD increases the interfacial instabilities and causes liquid extraction to the upper section of the tube. When the flow regime is initially annular flow, EHD increases the uniformity of the annular film by extracting liquid from the thicker liquid regions into the vapor core.Copyright

OSCILLATORY ENTRAINED DROPLET EHD TWO-PHASE FLOW

w In an experimental investigation of electrohydrodynamic tw phase flow, the application of a 60 Hz AC voltage potential convective boiling and condensation systems has led to a reg unique to any flow pattern previously observed @1–3#. It is an oscillatory flow, where droplets as large as 2 mm in diameter entrained in a vapor core that is surrounded by an annular liq film around the circumference of the tube and electrode, res bling a multi-layered annular flow. The droplets oscillate radia in the lower portion of the annulus at a frequency of appro mately 120 Hz, twice the frequency of the applied field, occasi ally being entrained by the inner or outer annular film. In additi to the droplet formation, small spouts or jets of liquid were o served on the upper half of the annular film surrounding the e trode. These spouts seemed to form randomly on the crest wave created by interfacial instabilities present in the film a would spray a fine mist into the upper portion of the vapor core shown in the upper figure. The size of the droplets, intensity motion, rate of deposition and the occurrence of spouts w highly dependent on the amplitude of the 60 Hz applied volta The interaction between the phases and the extremely high terfacial area, when coupled with the increased turbulent mix created by the oscillatory motion of the flow, led to significa enhancement of the overall Nusselt number ~;300 percent ! and overall pressure drop in both condensation and evaporation w applied under the appropriate conditions @1–3#. The frequency of

The Heat Transfer Characteristics of Exhaust Gas Recirculation (EGR) Cooling Devices

A one-dimensional steady state model was developed to predict the heat transfer performance of a shell (liquid)-and-tube (gas) heat exchanger used as a cooling device for exhaust gas recirculation (EGR) application where there is a significant temperature drop across the device. The predictions of the model results were compared with experimental measurements and the trends were found to be in good agreement for most of the transitional and turbulent regimes. The results showed that the exit gas temperature increases with increasing gas mass flow rate at fixed gas inlet temperature and coolant flow rate. It was also found that the exit gas temperature was essentially independent of the coolant flow rate for the typical operating range but did depend on the coolant inlet temperature. It was observed that the pressure drop across the cooling device was not a strong function of the gas inlet temperature. The heat-transfer effectiveness of the cooling device was found to slightly depend on the gas mass flow rate and inlet gas temperature. A preliminary analysis showed that fouling in the EGR cooling device can have a significant effect on both the thermal and hydraulic performance of the cooling device.Copyright

Maximum power point tracking for thermoelectric generators with high frequency injection

Thermoelectric Generators (TEG) can harvest a part of the thermal energy otherwise lost in the exhaust gases of vehicles and are combined with Maximum Power Point Tracking (MPPT) schemes to maximize the power output. This paper proposes a novel TEG MPPT scheme named High Frequency Injection (HFI) method. The method injects a high frequency voltage to the TEG and yields a power with a high frequency component. This component is demodulated and yields a signal proportional to the distance from the optimal operation point. The duty cycle setpoint is adjusted with a proportional-integral (PI) controller. The method is compared to the Perturb & Observe method using a drive cycle. Both show good results in terms of dynamic tracking of the optimal operation point. However, the HFI method is shown to be significantly more robust against sensor noise.

Mechanisms of electrohydrodynamic flow boiling heat transfer in coaxial flow channels of dielectric refrigerant R-134a

Experimental and numerical investigations have been conducted to study the mechanisms involved in dielectric refrigerant R-134a electrohydrodynamic (EHD) flow boiling in a concentric horizontal flow channel. Numerical calculations of the electric field distribution in two-phase flow with different gas-liquid distributions and interfacial geometries in a concentric electrode arrangement are conducted by a finite element method. The experiments conducted are performed at inlet qualities from 0 to 20%, mass fluxes from 100 to 500 kg/m/sup 2/s, heat flux from 10 kW/m/sup 2/ to 20 kW/m/sup 2/ and applied voltage from 0 to 10 kV. It was found that the application of an electrohydrodynamic body force to an R-134a evaporator can result in a significant augmentation of heat transfer that may be partially explained by the numerically simulated electric field profiles near gas-liquid interfaces.

The effect of orientation on U-shaped grooved and sintered wick heat pipes

The thermal management system for a compact sealed computer is studied here. The system consists of two heat spreaders that are connected by two U-shaped heat pipes. The heat pipes are used to transport the heat directly from the chipset to the finned case wall where it is dissipated to the ambient by natural convection. In this study, the effect of orientation on the performance of the U shaped heat pipe was determined. Temperature difference and thermal resistance were measured for a range of heat load values ranging from 0 W to 45 W per heat pipe. Heat pipes with an eccentric sintered wick structure and with an axial groove wick structure were tested. The orientation was found to have a significant effect on the axial groove wick heat pipe performance with a much smaller effect on the sintered wick heat pipe performance. The experimental results were in good agreement with that predicted from the basic governing equations of heat pipes.