# Spatially resolved temperature measurement in microchannels

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## Abstract

Non-intrusive local temperature measurement in convective microchannel flows using infrared (IR) thermography is presented. This technique can be used to determine local temperatures of the visualized channel wall or liquid temperature near this wall in IR-transparent heat sinks. The technique is demonstrated on water flow through a silicon (Si) microchannel. A high value of a combined liquid emissivity and substrate overall transmittance coupled with a low uncertainty in estimating this factor is important for quantitative temperature measurement using IR thermography. The test section design, and experimental and data analysis procedures that provide increased sensitivity of the detected intensity to the desired temperature are discussed. Experiments are performed on a 13-mm long, 50 μm wide by 135 μm deep Si microchannel at a constant heat input to the heat sink surface for flow rates between 0.6 and 1.2 g min^{−1}. Uncertainty in fluid temperature varies from a minimum of 0.60°C for a Reynolds number (Re) of 297 to a maximum of 1.33°C for a Re of 251.

### Keywords

Microchannels Microscale heat transfer Infrared thermography Spatially resolved Temperature measurement Single-phase flows### List of symbols

*a*absorption coefficient (cm

^{−1})*D*_{h}channel hydraulic diameter (m)

*e*radiant energy flux (W m

^{−2})*G*radiant flux incident on the heat sink (W m

^{−2})*H*height of the channel (m)

*h*local heat transfer coefficient (W m

^{−2}K^{−1})*i*location index in channel direction (

*x*direction)*j*location index in a cross-direction to channel (

*y*direction)*i*′location index in detector array corresponding to

*i**j*′location index in detector array corresponding to

*j*- \( \ifmmode\expandafter\dot\else\expandafter\.\fi{m} \)
mass flow rate (kg s

^{−1})*n*refractive index

- Nu
Nusselt number \( ({\text{Nu}} = h\,D_{{\text{h}}}/k) \) (dimensionless)

*q*″heat flux to the heat sink (W m

^{−2})- Re
Reynolds number \( ({\text{Re}} = V\,D_{{\text{h}}} /\nu ) \) (dimensionless)

- \( \ifmmode\expandafter\bar\else\expandafter\=\fi{R} \)
total reflectance

*S*radiation path length (m)

*t*thickness of the medium (cm)

*T*_{f}fluid temperature (°C)

*V*channel mean velocity (m s

^{−1})*W*width of the channel (m)

*x*_{i}distance of position

*i*from the channel entrance (m)

### Greek symbols

*Δ*incremental value

*ε*emissivity

*λ*wavelength

*μ*dynamic viscosity of fluid (Pa s)

*ν*kinematic viscosity of fluid (m

^{2}s^{−1})*ρ*surface reflectivity

*τ*_{f}transmittance of water

### Superscripts and subscripts

- b
blackbody

- cal
calibration

- det
detector

- e
emission

- f
fluid

- hsb
heat sink background

- lens
camera lens

- sens
sensed

- Si
Silicon

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