This second of two companion papers presents an analysis of sliding bubble and wall heat transfer parameters measured during subcooled boiling in a square, vertical, upward flow channel. Bubbles were generated only from a single nucleation site for better observation of both the sliding bubble characteristics and their impact on wall heat transfer through optical measurement techniques. Specific interests include: (i) bubbles departure and subsequent growth while sliding, (ii) bubbles release frequency, (iii) coalescence of sliding bubbles, (iv) sliding bubbles velocity, (v) bubbles size distribution and (vi) wall heat transfer influenced by sliding bubbles. The results showed that sliding bubbles involve two distinct growth behaviors: (i) at low mass fluxes, sliding bubbles grew fast near the nucleation site, subsequently shrank, and then grew again, (ii) as mass flux increased, however, sliding bubbles grew more steadily. The bubbles originating from the single nucleation site coalesced frequently while sliding, which showed close relation with bubbles release frequency. The sliding bubble velocity near the nucleation site consistently decreased by increasing mass flux, while the observation often became reversed as the bubbles slid downstream due to the effect of interfacial drag. The sliding bubbles moved faster than the local liquid (i.e., u r <0) at low mass flux conditions, but it became reversed as the mass flux increased. The size distribution of sliding bubbles followed Gaussian distribution well both near and far from the nucleation site. The standard deviation of bubble size varied insignificantly through sliding compared to the changes in mean bubble size.

This paper presents an experimental strategy to achieve the accurate wall temperature measurement using infrared (IR) thermometry with the enhanced flow visualization. Our particular interest is focused on the measurement of two-phase flow parameters in a convective boiling system which involves a large heated area. For the present application, the important issues such as the design of test section and material selection were discussed along with our decision-making process. Then, the IR-based temperature tracking algorithm was established based on the multi-layer heater wall design proposed. To apply this algorithm, however, the optical properties of materials must be identified first. Thus, the optical features of selected materials, e.g., soda-lime glass and indium-tin-oxide (ITO) were investigated and the measured values were validated through experiments. The wall temperature tracking algorithm on the proposed heater wall design was validated both for steadystate and transient conditions. Also, such algorithm was proved to be applicable for the heat flux measurement. Finally, the feasibility of the present approach was demonstrated through a subcooled flow boiling experiment. The results showed that both the hydrodynamic motions of bubble and the corresponding wall temperature can be captured with high fidelity using the measurement strategy presented, from which the interrelation between the sliding vapor bubbles and the wall heat transfer were discussed.

Laser induced fluorescence (LIF) is a non-intrusive optical technique that uses fluorescent dyes to measure whole-field fluid scalars such as temperature, concentration, pH, etc. LIF measurements' accuracy is strongly influenced by the fluorescent dye's behavior under different experimental conditions. In particular, ratiometric LIF thermometry accuracy depends on the correct selection of fluorescent dyes mixtures. Therefore, a thorough characterizations of fluorescent dyes is needed to obtain optimal mixtures and suitable optical configurations for given experimental conditions. This work presents the experimental characterization of fluorescein-27 (FL27) and rhodamine-B (RhB) mixtures to determine suitable aqueous solutions for ratiometric LIF thermometry. The mixtures' fluorescence emission intensity was measured with a spectrofluorometer, and the influence of concentration ratio (C(RhB)/C(FL27)), temperature, excitation wavelength (λ(ext)), and pH were analyzed. The results show that the temperature dependence of FL27 emission intensity changed from a negative to a positive value as the excitation wavelength increased. The temperature sensitivity (4.0% per °C) of RhB and FL27 mixture under 532 nm excitation wavelength was found to be higher than that of the commonly used mixture of RhB and Rh110 (2.0% per °C) at the same excitation wavelength. While the emission intensities of the dyes are sensitive to pH value, the temperature dependence is unaffected. The influence of concentration ratio on temperature sensitivity depends on both the detected bands of the emitted spectrum and the temperature; the concentration ratio should be selected based on the measured temperature scope. A new multicolor method or advanced two color method with high temperature sensitivity (6.0% or 10.0% per °C) is presented. This technique was specially developed to improve whole-field temperature measurements.

a b s t r a c tA novel experimental method and data acquisition/analysis strategy that ensures reliable measurements of various subcooled flow boiling parameters is discussed. In this study, all experiments were performed by keeping a single active nucleation site within the entire heated area in a square upward flow channel. This approach greatly facilitated the observation of bubble and wall heat transfer features in subcooled boiling flow.Vapor bubbles originating from the nucleation site were observed with both micro-and macroscopic views from high-speed cameras while corresponding wall temperature was measured by an infrared camera. This allowed simultaneously capturing various bubbles characteristics with multiple scales (both space and time) as well as their impact on wall heat transfer. In addition, effort s were made to characterize the observed boiling behavior with high statistical accuracy by analyzing numerous images taken at each test condition.This study proves that by taking the current strategy excellent repeatability and thus reliability can be achieved for a wide range of flow boiling parameters such as bubble size, bubble velocity, statistical distribution of bubble size and time-averaged wall heat transfer coefficients. Also, the major sources of uncertainty for each measurement are thoroughly investigated, from which the final uncertainties are determined. Overall, the present study suggests what we must concern to achieve truly reliable and useful data from any boiling experiments.

No abstract