Patrick J Geoghegan

Frost Growth CFD Model of an Integrated Active Desiccant Rooftop Unit

A frost growth model is incorporated into a Computational Fluid Dynamics (CFD) simulation of a heat pump by means of a user-defined function in a commercial CFD code. The transient model is applied to the outdoor section of an Integrated Active Desiccant Rooftop (IADR) unit in heating mode. IADR is a hybrid vapor compression and active desiccant unit capable of handling 100% outdoor air (dedicated outdoor air system) or as a total conditioning system, handling both outdoor air and space cooling or heating loads. The predicted increase in flow resistance and loss in heat transfer capacity due to frost build-up are compared to experimental pressure drop readings and thermal imaging. The purpose of this work is to develop a CFD model that is capable of predicting frost growth, a potentially valuable tool in evaluating the effectiveness of defrost-on-demand cycles.

Needs and Approaches for Novel Characterization of Direct Hybrid Fuel Cell/Gas Turbines

Solid oxide fuel cell (SOFC)/ gas turbine (GT) hybrid systems possess the capacity for unprecedented performances, such as electric efficiencies nearly twice that of conventional heat engines at variable scale power ratings inclusive of distributed generation. Additionally, these hybrids can have excellent operational flexibility with turndowns possibly as great as 85%. There are, however, developmental needs such as turbomachinery characterization and re-design. A leading example is that of greater propensity to have occurrences of stall-surge given the significantly different operating environment in contrast to conventional heat engines. Additionally, dynamic variation in power generation has to be done with significant a priori insight to avoid thermomechanical threats to cell stack and turbomachinery.State-of-the-art approaches involving hardware-in-the-loop simulation and, ultimately, additive manufacturing are being pursued to enable such characterization and re-design considerations given variable and dynamic operability requirements. Compressor performance in hybrid systems has been characterized at the United States National Energy Technology Laboratory (NETL), inclusive of a capability of feed forward hardware-in-the-loop simulation of hybrid systems under dynamic conditions and a capability of replacing turbine and compressor components at a relatively low cost. This paper highlights some of the simulation results, and the net result is an approach that addresses hybrid system developmental needs for accommodating generation transients.Copyright

Neutron Imaging of a Two-Phase Refrigerant Flow

Void fraction remains a crucial parameter in understanding and characterizing two-phase flow. It appears as a key variable in both heat transfer and pressure drop correlations of two-phase flows, from the macro to micro-channel scale. Void fraction estimation dictates the sizing of both evaporating and condensing phase change heat exchangers, for example. In order to measure void fraction some invasive approach is necessary. Typically, visualization is achieved either downstream of the test section or on top by machining to expose the channel. Both approaches can lead to inaccuracies. The former assumes the flow will not be affected moving from the heat exchanger surface to the transparent section. The latter distorts the heat flow path.Neutron Imaging can provide a non-invasive measurement because metals such as Aluminum are essentially transparent to neutrons. Hence, if a refrigerant is selected that provides suitable neutron attenuation; steady-state void fraction measurements in two-phase flow are attainable in-situ without disturbing the fluid flow or heat flow path.Neutron Imaging has been used in the past to qualitatively describe the flow in heat exchangers in terms of maldistributions without providing void fraction data. This work is distinguished from previous efforts because the heat exchanger has been designed and the refrigerant selected to avail of neutron imaging.This work describes the experimental flow loop that enables a boiling two-phase flow; the heat exchanger test section and downstream transparent section are described. The flow loop controls the degree of subcooling and the refrigerant flowrate. Heating cartridges embedded in the test section are employed to control the heat input. Neutron-imaged steady-state void fraction measurements are captured and compared to representative high-speed videography captured at the visualization section. This allows a qualitative comparison between neutron imaged and traditional techniques. The measurements are also compared to correlations in the literature.Preliminary void fraction images from a macro-channel flow are presented, consisting of 1 channel, 4mm wide, 4mm high and 83.32mm long. Flow regime identification is examined.The experiments were conducted at the High Flux Isotope Reactor (HFIR) Cold Guide 1D neutron imaging facility at Oak Ridge National Laboratory, Oak Ridge, TN, USA.Copyright

Simulation and Mockup of SNS Jet-Flow Target With Wall Jet for Cavitation Damage Mitigation

Pressure waves created in liquid mercury targets at the pulsed Spallation Neutron Source (SNS) at Oak Ridge National Laboratory induce cavitation damage on the stainless steel target vessel. The cavitation damage is thought to limit the lifetime of the target for power levels at and above 1 MW. Severe through-wall cavitation damage on an internal wall near the beam entrance window has been observed in spent-targets. Surprisingly though, there is very little damage on the walls that bound an annular mercury channel that wraps around the front and outside of the target. The mercury flow through this channel is characterized by smooth, attached streamlines. One theory to explain this lack of damage is that the uni-directional flow biases the direction of the collapsing cavitation bubble, reducing the impact pressure and subsequent damage. The theory has been reinforced by in-beam separate effects data. For this reason, a second-generation SNS mercury target has been designed with an internal wall jet configuration intended to protect the concave wall where damage has been observed. The wall jet mimics the annular flow channel streamlines, but since the jet is bounded on only one side, the momentum is gradually diffused by the bulk flow interactions as it progresses around the cicular path of the target nose. Numerical simulations of the flow through this jet-flow target have been completed, and a water loop has been assembled with a transparent test target in order to visualize and measure the flow field. This paper presents the wall jet simulation results, as well as early experimental data from the test loop.Copyright