Previous projects

Enhancement of ethanol boiling using novel surface engineering

Due to the significantly reduced boiling point, organic fluids such as ethanol provide an attractive alternative to water as the working fluid in two-phase thermal management systems for high-heat-flux applications. The state-of-the-art boiling surface design for ethanol involve mainly surface structure engineering. Here we adopt the approach of surface wettability patterning in enhancing ethanol boiling. The coating of (P(FAC8-co-DOPAm)) modified with halloysite nanotubes show strong repellency against highly wetting ethanol. Its deposition on a polished copper surface led to a clear biphilic pattern of alternating hydrophobicity and hydrophilicity. Saturated boiling of ethanol was shown to improve considerably on the biphilic surfaces, which was attributed to more ordered bubble generation and growth. The optimal heat transfer performance—more than 300% enhancement compared with Rohsenow’s correlation at 20 K superheating—was obtained on the D2p5.5 surface with a pitch/diameter ratio around 2.5. Moreover, the onset of nucleate boiling (ONB) was lowered by almost 40%

[B. Shen, Takeshi Hamazaki, Wei Ma, Naoki Iwata, Sumitomo Hidaka, Astushi Takatahara, Koji Takahashi, and Yasuyuki Takata, “Enhanced pool boiling of ethanol on wettability-patterned surfaces”, Applied Thermal Engineering, Vol. 149 (2019), pp. 925-331]

Investigation of contact-line dynamics in saturated boiling on heterogeneous surfaces at different pressures

Boiling tends to suffer from inefficient intermittent cycles of bubble generation under subatmospheric conditions, which is responsible for particularly harmful deterioration of the heat transfer rate. The transition to intermittent boiling can be effectively delayed, but not completely eliminated, on surfaces with mixed wettabilities (namely, biphilic). In this study, we investigate the bubble dynamics on a single hydrophobic PTFE (polytetrafluoroethylene) spot in reduced-pressure boiling. The rest of the surface was plain copper surface polished to a mirror finish. The high-speed visualization reveal an interesting transition in bubble departure mechanism from the surface tension-dominated behavior to the one that is more driven by hydrodynamic drag force, which depends on the particular pinning state of the bubble contact line at the wettability divide. Based on the force-balance argument, a simple model is derived to map the contact-line traversing dynamics across the border between the hydrophobic and hydrophilic surfaces during bubble expansion. The increased contact-line mobility could cause total removal of any vapor residues from the hydrophobic spot and its deactivation as a viable nucleation site, leading ultimately to the emergence of intermittent boiling

[B. Shen, M. Yamada, T. Mine, S. Hidaka, M. Kohno, K. Takahashi, and Y. Takata, “Depinning of bubble contact line on a biphilic surface in subatmospheric boiling: Revisiting the theories of bubble departure”, International Journal of Heat and Mass Transfer, Vol.126 (2018), pp.715-720]

We employ the diffuse-interface approach to numerically study bubble expansion on a heating surface that consists of opposing wettabilities. The results show a dramatic shift in the dynamics of traversing contact line across the wettability divide under different gravities, which correspond to variable bubble growth rates.Only when the bubble growth becomes sufficiently weakened at high gravity does the contact line get slowed down drastically to the point of being nearly immobilized at the edge of the hydrophilic surface. The following bubble expansion, which faces strong limitations in the direction parallel to the surface, features a consistent apparent contact angle around 66.4^o, regardless of the wettability combination. A simple theoretical model based on the force balance analysis is proposed to describe the physical mechanism behind such a dramatic transition in the contact-line behavior

[B. Shen, J. Liu, G. Amberg, M. Do-Quang, J. Shiomi, K. Takahashi, and Y. Takata,“Contact-line behavior in boiling on a heterogeneous surface: Physical insights from diffuse-interface modeling”, Physical Review Fluids, Vol. 5 (2020), 033603]

Elucidation of the effect of dissolved gas on boiling on biphilic surfaces

For two-phase cooling schemes for computer components, quick activation of nucleate boiling helps safeguard the electronic device from thermal shocks associated with wild surface temperature excursions at boiling incipience. In this work, we report unambiguous evidence that unusual bubble generation at extremely low temperatures—even below the boiling point—can be induced by a significant presence of dissolved gas attracted by surface hydrophobicity. By depositing an array of hydrophobic PTFE (polytetrafluoroethylene) spots on the hydrophilic TiO₂ substrate, subcooled boiling of water in the open system (which is supposed to contain trace amounts of dissolved gas) could be significantly enhanced, with the onset of nucleate boiling (ONB) falling clearly below the saturation temperature. After eliminating the effect of dissolved gas in the closed system, the results show that nearly all the heat transfer gains in the low-superheat region has vanished. The sharp contrast of bubble dynamics between the mode of gassy boiling in the open system and the mode of gas-depleted boiling in the closed system is well captured by the numerical simulations using the diffuse-interface method

[B. Shen, M. Yamada, S. Hidaka, J. Liu, J. Shiomi, G. Amberg, M. Do-Quang, M. Kohno, K. Takahashi, and Y. Takata, “Early onset of nucleate boiling on gas-covered biphilic surfaces”, Scientific Reports, Vol. 7 (2017), pp.2036]

Direct numerical simulation of natural convection in supercritical fluids

Due to the peculiar thermophysical properties near the critical point (CP), thermoconvection in a supercritical fluid is unique for its intensity and complex flow patterns. In this paper, we numerically study the development of three-dimensional buoyant flow in a rectangular cavity filled with near-critical N₂. The hydrodynamic model modified by the low-Mach-number approximation is discretized and solved by an in-house multigrid SIMPLE computational code. Different convective regimes have been obtained under different heating arrangements. (i) In the case of full bottom heating, massive asymmetric thermal plume flow rising from the bottom wall quickly turn into an irregular pattern of large-scale turbulent circulation. (ii) The flow structure generated by a long heating strip embedded in the bottom wall (i.e., ½ bottom heating) consists of continual merger of thermal plumes, thanks to particularly violent bulk recirculation induced by enlarged compressibility close to the CP. (iii) A large column of hot fluid can be seen ascending from the finite-sized heat source at the center of the cavity bottom (i.e., ¼ bottom heating). The strong equivalency among different near-critical fluids is also demonstrated by replacing N₂ with CO₂ with a comparable degree of critically

[B. Shen and P. Zhang, “Three-dimensional thermoconvection from a non-uniformly heated plate near the liquid-vapor critical point”, International Journal of Thermal Sciences, Vol. 89 (2015), pp.136-153]

Study of thermoacoustic interaction near the liquid-vapor critical point

Near the liquid-vapor critical point (CP), thermal disturbances can generate sounds. Such sound waves, whose propagation is isentropic in nature, then raise the bulk temperature and pressure gradually as they are transmitted in the fluid, constituting the so-called “piston effect” (PE) that is responsible for accelerated critical thermalization. So far, there are few studies devoted to direct examination of the thermoacoustic process very close to the CP, which is considered to be the driving mechanism behind the PE. Here, we aim to illustrate the evolution of thermoacoustic waves across a wide range of distances to the CP (defined using the reduced temperature ε) by numerically solving the governing differential equations. The results show markedly different behaviors between the cases of (linear) internal-source heating and (nonlinear) boundary heating. In the former case, homogeneous thermoacoustic waves are induced by the heat input. On the other hand, under the nonlinear thermal disturbance, pronounced distortions to the wavefront profile result from varying distances to the CP. In later times, the simulations for both N₂ and CO₂ show gradual convergence between these two cases in terms of the energy yield of the thermomechanical conversion

[B. Shen and P. Zhang, “Thermoacoustic waves along the critical isochore”, Physical Review E, Vol. 83 (2011), pp.011115]

Dual-phase modeling of non-Fourier heat conduction

In this study, a number of notable physical anomalies concerning non-Fourier heat conduction under the dual-phase-lag (DPL) model are observed and investigated. It is found that, during the transient heat transfer process, the over-diffusion mode predicts a “hyper-active” to “under-active” transition in thermal behavior. The main cause behind it lies in the time-varying effect of τ_T (the phase lag of the temperature gradient) on the thermal response. Also, change of polarity in reflected thermal waves can be observed when a constant-temperature boundary is involved, which hints that a heating process may be followed by a spontaneous cooling effect. A fairly strong connection is present between the τ_T-induced dispersive effect and an unusual thermal accumulation phenomenon in an on–off periodic heating process. Furthermore, a paradox involving a moving medium is detected in the DPL model, which can be solved by replacing the temporal partial derivatives in the DPL equation with the material derivatives.

[B. Shen and P. Zhang, “Notable physical anomalies manifested in non-Fourier heat conduction under the dual-phase-lag model”, International Journal of Heat and Mass Transfer, Vol. 51 (2008), pp.1713-1727]