Experimental and simulation results related to the analysis of the heat transfer features for gas flows under rarefied conditions through microstructure devices will be presented here. The mostly used devices for similar studies are simple commercially available micro capillaries made of stainless steel. The geometrical precision of manufacturing of such capillaries may differ drastically, which results in more or less strong deviations of experimental results from pre- calculations using classical numerical approaches. In addition, in rarefied conditions the dynamic and thermal characteristics are strongly dependent on the surface features of the walls which influence the values assumed by the accommodation coefficients used in many simplified models for the slip-flow regime. In order to compare the experimental results with the theoretical predictions a more precise qualification of the interaction of surfaces to the gas flow and the heat transfer is needed. For this objective, a specific microstructure device has been designed and developed which allows to vary the integrated microstructure as well as to implement an innovative sensor array providing direct access to the gas flow through a single microchannel avoiding major disturbances on the flow. The device consists of a conventionally dimensioned frame with an exchangeable microchannel section containing a single channel being 400μm wide and around 100μm deep. The microchannel section was manufactured from different materials (stainless steel, copper and PEEK polymer) to provide different thermal properties and different surface roughness at the microchannel walls. For the stainless steel sections, two different manufacturing approaches have been chosen to change also the cross sectional shape of the microchannel from rectangular (obtained by precision machining) to semi-elliptic (obtained by wet chemical etching). Surface roughness of each microchannel was characterised using different methods. The new developed sensor allows to measure the temperature evolution of the gas flow along the microchannel. In accordance to the results obtained with some capillaries and a single microchannel, a multi-microchannel heat exchanger was designed and manufactured. This design allows the arrangement of a freely selectable number of microchannel foils. The microchannel arrangement can be changed from crossflow to counter-current or co-current flow. The device is manufactured from PEEK polymer to reduce the heat losses to the environment and the negative effects due to conjugate wall-fluid heat transfer. It enables the test of gas flows from rarefied conditions up to highly turbulent flows under high pressure of up to 40 bar. Due to reasons of mechanical stability, a special design for the inlet and outlet voids had to be manufactured, showing numerous micro pillars within the void. Preliminary numerical results for the flow distribution of the gas in the inlet and outlet voids will be provided. The projects are still ongoing, thus, experimental results obtained with the multi-microchannel device will be shown. The research leading to these results has received funding from the Seventh Framework Program under grant agreement PITN-GA-2008-215504, "Gas Flows in Micro Electro Mechanical Systems.
Brandner J.J., Vittoriosi A., Yang Y., Morini G.L. (2012). Microstructure Devices for Heat Transfer Studies in Gas Flows. Zaragoza : s.n.
Microstructure Devices for Heat Transfer Studies in Gas Flows
YANG, YAHUI;MORINI, GIAN LUCA
2012
Abstract
Experimental and simulation results related to the analysis of the heat transfer features for gas flows under rarefied conditions through microstructure devices will be presented here. The mostly used devices for similar studies are simple commercially available micro capillaries made of stainless steel. The geometrical precision of manufacturing of such capillaries may differ drastically, which results in more or less strong deviations of experimental results from pre- calculations using classical numerical approaches. In addition, in rarefied conditions the dynamic and thermal characteristics are strongly dependent on the surface features of the walls which influence the values assumed by the accommodation coefficients used in many simplified models for the slip-flow regime. In order to compare the experimental results with the theoretical predictions a more precise qualification of the interaction of surfaces to the gas flow and the heat transfer is needed. For this objective, a specific microstructure device has been designed and developed which allows to vary the integrated microstructure as well as to implement an innovative sensor array providing direct access to the gas flow through a single microchannel avoiding major disturbances on the flow. The device consists of a conventionally dimensioned frame with an exchangeable microchannel section containing a single channel being 400μm wide and around 100μm deep. The microchannel section was manufactured from different materials (stainless steel, copper and PEEK polymer) to provide different thermal properties and different surface roughness at the microchannel walls. For the stainless steel sections, two different manufacturing approaches have been chosen to change also the cross sectional shape of the microchannel from rectangular (obtained by precision machining) to semi-elliptic (obtained by wet chemical etching). Surface roughness of each microchannel was characterised using different methods. The new developed sensor allows to measure the temperature evolution of the gas flow along the microchannel. In accordance to the results obtained with some capillaries and a single microchannel, a multi-microchannel heat exchanger was designed and manufactured. This design allows the arrangement of a freely selectable number of microchannel foils. The microchannel arrangement can be changed from crossflow to counter-current or co-current flow. The device is manufactured from PEEK polymer to reduce the heat losses to the environment and the negative effects due to conjugate wall-fluid heat transfer. It enables the test of gas flows from rarefied conditions up to highly turbulent flows under high pressure of up to 40 bar. Due to reasons of mechanical stability, a special design for the inlet and outlet voids had to be manufactured, showing numerous micro pillars within the void. Preliminary numerical results for the flow distribution of the gas in the inlet and outlet voids will be provided. The projects are still ongoing, thus, experimental results obtained with the multi-microchannel device will be shown. The research leading to these results has received funding from the Seventh Framework Program under grant agreement PITN-GA-2008-215504, "Gas Flows in Micro Electro Mechanical Systems.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
