(97b) Experimental Investigation of Flow Boiling Heat Transfer in a Microchannel by Using Infrared Thermography

Heat transfer during flow boiling in small channels has received growing attention in recent years due to their high potential applications in many fields. Compact heat exchangers can be employed in mobile energetic systems like fuel cells and also in chemical process engineering, where many of the reactions take place in the gas phase and the vaporization of the usually liquid reactants is necessary. Although the number of investigations has increased rapidly in recent years, some aspects still remain unclear. For example, no generally accepted correlations are available for calculation of heat transfer coefficients and pressure drop. Also, the lower limit of the hydraulic diameter, for which classical correlations for conventional channels remain applicable, is still to be identified. However and thanks to the support of high resolution visualizations, some light has been shed on the problem of boiling in small channels. Nucleate boiling seems to play an important role, either in form of elongated bubbles, which develop into slugs and fill the entire channel, or bubbles which nucleate in the thin liquid film near the wall. Also some flow pattern maps have been developed from the experimental visualizations. The present study focuses on the experimental investigation of boiling heat transfer in a single rectangular microchannel. In contrast to many of the available studies, where thermocouples have been used to determine the temperature on the outer surface of the channels, infrared thermography is employed in the present work. This measurement method allows for the continuous and contactless measurement of the temperature field on the channel wall. A high spatial and temporal resolution permits an analysis of the unsteady behavior of boiling in small channels, which has been often reported in literature. In order to obtain reliable values of the heat transfer coefficient, the wall temperature will be averaged over a determined period of time. This is set to be the minimum time period over which the average wall temperatures remain constant. The present study aims to contribute to a better understanding of flow boiling in narrow channels. Water, ethanol and n-hexane are used as working fluids. Experiments are carried out for mass fluxes between 50 and 500 kg/m²s and heat fluxes up to 400 kW/m². The experimental setup can be described as follows: the working fluid flows from a container through a pump and a heat exchanger, where the desired inlet temperature is set. Afterwards, it passes through a mass flow meter and enters the test section. The fluid flows upwards and the test section, which is made of the nickel alloy Inconel 600, is electrically heated. After leaving the test section, the fluid condenses and returns to the reservoir. Once both mass and heat fluxes are set and after the desired operating conditions are reached, the temperature distribution of the channel is recorded by the IR-Camera. The data recording is performed by a ThermaCam®SC3000, manufactured by FLIR Systems. A high emissivity of the scanned surface is required to reduce the uncertainty of the temperature measurement. Therefore, the channel is coated with a very thin black lacquer, which provides an emissivity of approximately 0.95. The emissivity is a strong function of the structure and temperature of the surface, and, since the knowledge of the emissivity of the surface is essential to obtain reliable results, a calibration is necessary to describe these dependencies and correct the measured temperatures. The temperature field on the channel surface is recorded at a frequency of 150 Hz for 25 s. An analysis area is positioned on each thermographic picture, so that the spatio-temporal temperature distribution is available. An unsteady analysis is carried out first and then adequate values of the wall temperature are selected in order to obtain reliable heat transfer coefficients. As far as the experimental results are concerned, high amplitude oscillations of the outer wall temperature are registered at low vapor qualities in the case of water. The amplitude decreases with increasing vapor quality and the frequency increases. However, no characteristic oscillation frequency can be observed. A different unsteady behavior is registered for ethanol. For a constant value of the mass flux, both oscillation amplitude and frequency depend on the applied heat flux. For high heat fluxes, nearly periodic oscillations of the wall temperature are registered. This change in the unsteady behavior depending on the applied heat and mass fluxes can also be appreciated in the trend of the heat transfer coefficient, which is calculated by using time averaged values of the wall temperature. Finally, the boiling process of n-hexane is observed to occur in a more stable way. Only low amplitude and frequency oscillations are registered. The higher amplitudes are observed for water under the given experimental conditions. Since these oscillations are measured at the outer wall of the channel, it is also necessary to study the effect of the wall thickness on both attenuation of the amplitude and phase shift. Therefore, the computer software FLUENT® is used to simulate the inner wall temperatures from the experimental values. This unsteady analysis aims to contribute to the determination of reliable values of the heat transfer coefficient. Thus, as already mentioned, a minimum measuring time is established and the time averaged wall temperatures are used in the data reduction. The inner wall temperatures are calculated by considering one dimensional heat conduction through the wall. The fluid temperature is obtained from an energy balance between inlet and the desired axial position in the single phase region and it is set to be the saturation temperature in the two phase region. The saturation temperature along the channel is a function of the local pressure. The time averaged heat transfer coefficients are represented over both length and vapor quality. In every case, the heat transfer coefficient increases strongly in the region of subcooled boiling, for values of quality below 0. Afterwards, a strong decrease is observed. This decrease is more pronounced in the case of water. This seems to be related to the high amplitude oscillations registered in this region. It is worth to note that the bubble departure diameter of water under the given conditions is substantially higher than that of ethanol or n-hexane. After this first decrease, two different trends are registered. For saturated flow boiling of water, the heat transfer coefficient decreases with increasing vapor quality and increases with increasing heat flux. The behavior has already been reported in literature for water. On the other hand, in the case of ethanol and n-hexane, the heat transfer coefficient can either increase or decrease depending on the applied mass and heat fluxes. This change in the trend of the heat transfer coefficient has also already been reported in literature and a dimensionless number, derived from the boiling number was proposed to predict the relative dominance of the acting forces. This proposal, as well as some available correlations developed for boiling in small channels, is taken into account in this work for the evaluation of the experimental results