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Flow boiling heat transfer is a promising technology for thermal management. In this paper, flow boiling heat transfer characteristics in a rectangular vertical flow channel are experimentally and numerically investigated at low flow rate. Visualization results provide guidance for the prediction of flow patterns. The flow boiling correlation at low flow rates is improved with an error of less than 30%. Numerical results reveal vortices enhanced local heat transfer. Absolute vorticity is introduced to quantify the secondary flow. The results indicate that the secondary flow intensity increases with heat flux, verifying that boiling flow produces the local convection enhancement. [ABSTRACT FROM AUTHOR]

Boiling flow presents a significant concern, especially when a liquid surpasses its boiling point, potentially leading to catastrophic consequences. This research utilizes a two-phase code in the OpenFOAM software to investigate bubble formation during flow boiling. The well-established empirical models for calculating wall heat components were selected based on the operating conditions. The study incorporates experimental data from high-pressure boiling flow (10–30 bars) with variable properties of refrigerant R-12. The predictions reveal underpredictions in void fraction and liquid temperature compared to experimental observations. Significantly, the impact of the subcooling degree on void fraction behaviour is emphasized, and a potential underprediction of the evaporation portion is highlighted, particularly near the wall. Challenges in modelling bubble size distribution are evident through discrepancies in bubble diameter and velocity data, indicating the necessity for further advancements in the code. In summary, this numerical study provides valuable insights into the intricate dynamics of high-pressure subcooled boiling flow, especially when considering variable working fluid properties. Future efforts will focus on refining models for nucleation site density, bubble departure size, and lift-off frequency to enhance prediction accuracy.

Two-phase flow loop cooling system is required for increasing heat transfer rate, heat transport distance, and cooling heat flux in thermal control of space structures. The understandings on thermo-fluid dynamics of gas-liquid two-phase flows with phase change under microgravity is necessary for the system design. To clarify the phenomena and verify the operation of two-phase flow loops under microgravity condition, two-phase flow loop experiments were conducted as a JAXA project named TPF experiment onboard the International Space Station (ISS). The loop was a pump-driven loop and the pressure was maintained by an accumulator. Perfluorohexane was used as the working fluid because of low latent heat of vaporization and moderate boiling point under the restriction of power input and for the safe operation. The loop had two kinds of heating section, such as a copper heating tube and a transparent glass heating tube, and a condenser. Flow observation sections made of transparent polycarbonate resin were installed just downstream of each heating tube in order to observe and measure the interface structure of boiling two-phase flow in detail. In this report, the design concept and system characteristics of the experimental loop and the results of the gas-liquid two-phase flow measurements in the flow observation section are presented. [ABSTRACT FROM AUTHOR]

• Advanced heat transfer tubes with dimple and sandblast surfaces. • Two phase heat transfer characteristics of refrigerant R32 on the annulus side of the tubes. • Influence of refrigerant mass flux and vapor quality on heat transfer characteristics. Enhancing the heat transfer performance of two-phase flow in heat transfer tubes is of paramount importance. In pursuit of this goal, the research group fabricated heat transfer tubes featuring both microscopic and macroscopic surfaces containing staggered arrays of three-dimensional dimples (depth of 2 mm, diameter of 3 mm, and spaced at intervals of 10 mm), petal-shaped protrusions (depth of 850 μm, diameter of 980 μm, and spaced at intervals of 50 μm), and sandblast structure (diameter ranging from 5 to 10 μm and depth of approximately 1 μm). Experiments were carried out to examine boiling and condensation two-phase flow heat transfer in annulus outside the tubes, including smooth tube, sandblast tube (E1), dimple tube (E2), and sandblast/dimple composite surface tube (E3), all constructed from stainless steel with a tube diameter of 19.05 mm. Both heat transfer coefficient (HTC) and frictional pressure drop exhibited a positive correlation with mass fluxes. In condensation, HTC increased as vapor quality increased. Sandblasting of the dimple surface had a detrimental effect on HTC, with the HTC of the E3 tube falling between that of the E2 and E1 tubes. In flow boiling, sandblasting the dimple structure's surface further enhanced the vaporization core, resulting in the E3 tube displaying the highest HTC, benefiting from advantages of both sandblast and dimple structures. The effect of vapor quality on HTC was found to be minor at low mass fluxes, while at high mass fluxes, HTC increased initially and then decreased as vapor quality increased. [ABSTRACT FROM AUTHOR]

The rectangular channel is of particular interest for nuclear reactors due to its compact structure and high heat transfer efficiency. Two-phase flow characteristics in rectangular channels are different from those in conventional circular channels, which is important for the design and safety analysis of reactors. A subcooled boiling flow experiment was conducted using a single-side heated vertical rectangle channel at various system pressures of 0.80 MPa–1.80 MPa. Local two-phase parameters, namely void fraction, interfacial area concentration, bubble interfacial velocity, and bubble chord length, were measured by a double-sensor conductivity probe, and flow visualization images were captured by a high-speed camera. The bubble distribution feature of subcooled boiling flow in rectangular channels is revealed in this study. A "W" type distribution of void fraction was observed in cap-bubbly flow and the interfacial area concentration has a similar distribution as the void fraction, which illustrates that bubbles gather at the center and sides of the channel. Different from the adiabatic flow, the system pressure is a positive factor for the void fraction which is observed in this experiment. Furthermore, the accuracy of void fraction prediction models is assessed based on the experimental data. A large deviation is observed between the predictive and the experimental void fraction, which is related to the single-side heating feature of the flow channel. [ABSTRACT FROM AUTHOR]

• Coupled continuity-IATE model predicts the growth in void and IAC in boiling flow. • Cap/slug bubbles are formed through the source terms, not flow-regime transition criteria. • The sensitivity to the boundary conditions and nucleation uncertainty is evaluated. • Improved model predictions of total void fraction compared to one-group models. The Two-Fluid Model is the backbone of thermal-hydraulics and system-analysis codes for nuclear design. The Two-Fluid Model tracks the transfer of conserved quantities—mass, momentum, and energy—without the need for bubble interface tracking. However, two-phase flows are characterized by their different flow regimes which change as more vapor is present in the flow, from bubbly flow to cap/slug flow to annular flow. The two-group Two-Fluid Model can track the progression of boiling flows beyond the bubbly region without the need for flow-regime maps. A two-group wall-boiling model is implemented and coupled with the two-group Interfacial Area Transport Equation. The resulting model is compared against experimental data and predicts the growth in cap/slug bubbles through interaction models rather than through flow-regime maps. The coupled model can predict the growth in void fraction and interfacial area concentration beyond the capability of the one-group model, demonstrating the applicability of a coupled two-group approach to modeling boiling flow. [ABSTRACT FROM AUTHOR]

It is key in several industrial applications to accurately describe rapid depressurization of liquid and dense phase states. Examples include refrigeration systems, nuclear reactor cooling and CO 2 capture and storage (CCS). It is expected that large-scale CO 2 pipeline transportation must be deployed as a vital part of reaching net zero emissions by 2050. During rapid depressurization of liquid-like CO 2 , boiling will in many cases occur out of equilibrium, at a lower pressure than the local saturation pressure. Capturing the non-equilibrium effects is necessary to predict outflow rates and the resulting pressure and temperature inside the pipe. In the present work, we quantify the non-equilibrium effects by studying a series of CO 2 pipe depressurization experiments from liquid-like states at initial temperatures from 10 °C to 40 °C. We compare the experimental results to predictions of the homogeneous equilibrium model (HEM) and a homogeneous relaxation-type non-equilibrium model (HRM*) where the mass-transfer rate from liquid to gas is tuned by a relaxation time. The relaxation time was found to decrease for increasing temperatures, and it was observed to be approximately 60 times longer for the coldest experiment than for the warmest one. [Display omitted] • We present pipe depressurization experiments of CO 2 at various initial temperatures. • The CO 2 was depressurized from supercritical pressures at a dense, liquid-like state. • We compare the results to the homogeneous equilibrium model and a relaxation model. • Shorter relaxation times were found for the higher-temperature experiments. • The relaxation time correlated with the scaled, relative initial entropy. [ABSTRACT FROM AUTHOR]