FLOW BOILING OF WATER AND n -HEPTANE IN MICRO CHANNELS

Flow boiling heat transfer characteristics of water and hydrocarbons in mini and microchannels are experimentally studied. Two different test section geometries are employed; a circular channel with a hydraulic diameter of 1,500 lm, and rectangular channels with height values of 300-700 lm and a width of 10 mm. In both facilities, the fluid flows upwards and the test sections, made of the nickel alloy Inconel 600, are directly electrically heated. Thus, evaporation takes place under the defined boundary condition of constant heat flux. Mass fluxes between 25 kg/m 2 s and 350 kg/m 2 s and heat fluxes from 20 kW/m 2 to 350 kW/m 2 at an inlet pressure of 0.3 MPa are examined. Infrared (IR) thermography is applied to scan the outer wall temperatures. These allow the identification of different boiling regions, boiling mechanisms, and the determination of the local heat transfer coefficients (HTC). Measurements are carried out in initial, saturated, and post-dryout boiling regions. The experimental results in the region of saturated boiling are compared with currently available correlations and with a physically founded model developed for convective boiling.

Section: Resultsmentioning

confidence: 98%

Section: List Of Symbols Amentioning

confidence: 98%

Section: Experimental Setup and Data Reductionmentioning

confidence: 99%

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Investigation of flow boiling in narrow channels by thermographic measurement of local wall temperatures

Díaz

Boye

Hapke

et al. 2005

“…There are numerous techniques currently being developed for temperature measurement at the microscale. IR thermography, (Hestroni et al 2003;Patil and Narayanan 2005;Hapke et al 2002), thermoreflectance (Christofferson et al 2001), Brownian motion of micro-PIV particles (Hohreiter et al 2002), and the use of thermochromic crystals (Chaudhari et al 1998;Fung et al 2006). To date, these techniques have not offered the potential for full field local measurement being restricted to surface measurements, volume emission effects, limited accuracy and for thermochromic crystals, the need for in situ calibration.…”

Section: Introductionmentioning

confidence: 97%

Full field measurement at the micro-scale using micro-interferometry

Garvey

Newport

Lakestani

et al. 2007

This paper presents micro-interferometry as a measurement technique to extract temperature profiles and/ or mass transfer gradients rapidly and locally in microdevices. Interferometry quantifies the phase change between two or more coherent light beams induced by temperature and/or mass concentration. Previous work has shown that temporal noise is a limiting factor in microscale applications. This paper examines phase stepping and heterodyne phase retrieval techniques with both CCD and CMOS cameras. CMOS cameras are examined owing to the high speed at which images can be acquired which is particularly relevant to heterodyne methods. It is found that heterodyne retrieval is five times better than phase stepping being limited to 0.01 rad or k/628. This is twice the theoretical limit of k/1,000. The technique is demonstrated for mixing in a T-junction with a 500 lm square channel and compared favourably to a theoretical prediction from the literature. Further issues regarding application to temperature measurements are discussed.

“…Most of the data presented were for flows in the turbulent or transitional regime, and agreed well with correlations in the literature. Hapke et al (2002) used IRT to examine the outside wall temperature distribution for flow boiling in rectangular microchannels and concluded that it is possible to detect the streamwise position of flow boiling regions using this technique. Muwanga and Hassan (2005) demonstrated the use of un-encapsulated thermochromic liquid crystal thermography to measure the outer surface temperature of a microtube.…”

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confidence: 99%

Spatially resolved temperature measurement in microchannels

Patil

Narayanan

2006

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 IRtransparent 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 lm wide by 135 lm deep Si microchannel at a constant heat input to the heat sink surface for flow rates between 0.6 and 1.2 g min À1 . 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 AE Microscale heat transfer AE Infrared thermography AE Spatially resolved AE Temperature measurement AE Single-phase flows List of symbols a absorption coefficient (cm À1 ) D h channel hydraulic diameter (m) e radiant energy flux (W m À2 ) G radiant flux incident on the heat sink (W m À2 ) H height of the channel (m) h local heat transfer coefficient (W m À2 K À1 ) 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 _ m mass flow rate (kg s À1 ) n refractive index Nu Nusselt number ðNu ¼ h D h =kÞ (dimensionless) q¢¢ heat flux to the heat sink (W m À2 ) Re Reynolds number ðRe ¼ V D h =mÞ (dimensionless) " 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 À1 ) W width of the channel (m) x i distance of position i from the channel entrance (m) Greek symbols D incremental value e emissivity k wavelength l dynamic viscosity of fluid (Pa s) m kinematic viscosity of fluid (m 2 s À1 ) q surface reflectivity s 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