Title:
Heat Transfer and Pressure Drop During Condensation of Refrigerants in Microchannels
Heat Transfer and Pressure Drop During Condensation of Refrigerants in Microchannels
Author(s)
Agarwal, Akhil
Advisor(s)
Garimella, Srinivas
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Abstract
Two-phase flow, boiling, and condensation in microchannels have received considerable
attention in the recent past due to the growing interest in the high heat
fluxes made possible by these channels. This dissertation presents a study on
the condensation of refrigerant R134a in small hydraulic diameter (100 < Dh
< 160 mm) channels. A novel technique is used for the measurement
of local condensation heat transfer coefficients in small quality increments,
which has typically been found to be difficult due to the low heat transfer
rates at the small flow rates in these microchannels. This method is used to
accurately determine pressure drop and heat transfer coefficients for mass
fluxes between 300 and 800 kg/m2-s and quality 0 < x <
1 at four different saturation temperatures between 30 and 60oC. The
results obtained from this study capture the effect of variations in mass flux,
quality, saturation temperature, hydraulic diameter, and channel aspect ratio
on the observed pressure drop and heat transfer coefficients. Based on the available
flow regime maps, it was assumed that either the intermittent or annular flow
regimes prevail in these channels for the flow conditions under consideration.
Internally consistent pressure drop and heat transfer models are proposed
taking into account the effect of mass flux, quality, saturation temperature, hydraulic
diameter, and channel aspect ratio. The proposed models predict 95% and 94% of
the pressure drop and heat transfer data within ±25%, respectively. Both
pressure drop and heat transfer coefficient increase with a decrease in
hydraulic diameter, increase in channel aspect ratio and decrease in saturation
temperature. A new non-dimensional parameter termed Annular Flow Factor is also
introduced to quantify the predominance of intermittent or annular flow in the
channels as the geometric parameters and operating conditions change. This
study leads to a comprehensive understanding of condensation in microchannels
for use in high-flux heat transfer applications.
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Date Issued
2006-11-20
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Resource Type
Text
Resource Subtype
Dissertation