The gas temperature in protoplanetary disks (PPDs) is determined by a combination of irradiation heating and accretion heating, with the latter conventionally attributed to turbulent dissipation. ...However, recent studies have suggested that the inner disk (a few au) is largely laminar, with accretion primarily driven by magnetized disk winds, as a result of nonideal magnetohydrodynamic (MHD) effects from weakly ionized gas, suggesting an alternative heating mechanism by Joule dissipation. We perform local stratified MHD simulations including all three nonideal MHD effects (ohmic, Hall, and ambipolar diffusion) and investigate the role of Joule heating and the resulting disk vertical temperature profiles. We find that in the inner disk, as ohmic and ambipolar diffusion strongly suppress electrical current around the midplane, Joule heating primarily occurs at several scale heights above the midplane, making the midplane temperature much lower than that with the conventional viscous heating model. Including the Hall effect, Joule heating is enhanced/reduced when the magnetic fields threading the disks are aligned/anti-aligned with the disk rotation, but it is overall ineffective. Our results further suggest that the midplane temperature in the inner PPDs is almost entirely determined by irradiation heating, unless viscous heating can trigger thermal ionization in the disk innermost region to self-sustain magnetorotational instability turbulence.
Abstract
The temperature structure of protoplanetary disks provides an important constraint on where in the disks rocky planets like our own form. Recent nonideal magnetohydrodynamical (MHD) ...simulations have shown that the internal Joule heating associated with magnetically driven disk accretion is inefficient at heating the disk midplane. A disk temperature model based on the MHD simulations predicts that in a disk around a solar-mass young star, the water snow line can move inside the current Earth’s orbit within 1 Myr after disk formation. However, the efficiency of the internal Joule heating depends on the disk’s ionization and opacity structures, both of which are governed by dust grains. In this study, we investigate these effects by combining the previous temperature model for magnetically accreting disks with a parameterized model for the grain size and vertical distribution. Grain growth enhances the gas ionization fraction and thereby allows Joule heating to occur closer to the midplane. However, growth beyond 10
μ
m causes a decrease in the disk opacity, leading to a lower midplane temperature. The combination of these two effects results in the midplane temperature being maximized when the grain size is in the range 10–100
μ
m. Grain growth to millimeter sizes can also delay the snow line’s migration to the 1 au orbit by up to a few million years. We conclude that accounting for dust growth is essential for accurately modeling the snow line evolution and terrestrial planet formation in magnetically accreting protoplanetary disks.
Abstract
The low water content of the terrestrial planets in the solar system suggests that the protoplanets formed within the water snow line. Accurate prediction of the snow line location moving ...with time provides a clue to constraining the formation process of the planets. In this paper, we investigate the migration of the snow line in protoplanetary disks whose accretion is controlled by laminar magnetic fields, which have been proposed by various nonideal magnetohydrodynamic (MHD) simulations. We propose an empirical model of the disk temperature based on our nonideal MHD simulations, which show that the accretion heating is significantly less efficient than that in turbulent disks, and calculate the snow line location over time. We find that the snow line in magnetically accreting laminar disks moves inside the current Earth’s orbit within 1 Myr after star formation, whereas the time for the conventional turbulent disk is much longer than 1 Myr. This result suggests that either the rocky protoplanets formed in such an early phase of the disk evolution, or the protoplanets moved outward to the current orbits after they formed close to the protosun.
•Microlayer characteristics were studied in whole heat flux range of nucleate boiling.•Existence of microlayer at CHF condition was experimentally verified.•Maximal microlayer radius decreased with ...the increase of heat flux until CHF.•Bubble size in macrolayer was limited due to absorption by the vapor lump.•There was few dependency of initial microlayer thickness distribution on heat flux.
During nucleate boiling, a thin liquid film (microlayer) is formed beneath a boiling bubble when the bubble undergoes rapid growth/expansion. The linear distribution of the microlayer has been previously confirmed, and its crest shape has been observed in the isolated bubble region of nucleate boiling. However, the microlayer behavior in larger heat flux regions up to critical heat flux has not yet been elucidated. In this study, to further understand the microlayer structure in the whole heat flux range of nucleate boiling, microlayer configuration was measured using laser interferometry. Water was adopted as the test fluid, and it is confirmed that the microlayer can be observed over a whole range of nucleate boiling containing the critical heat flux point. It is also confirmed that the deformation of the microlayer from axisymmetric shape was caused by complicated, irregular bubble motions such as bubble coalescence, which was observed for relatively higher heat flux. The crest shape of the microlayer, which appears near the periphery of its maximum diameter under relatively smaller heat flux, was not observed at a relatively higher heat flux. Finally, it is confirmed that heat flux does not obviously influence the thickness distribution of the initial microlayer.
Abstract
We performed synthetic observations of the Ulrich, Cassen, and Moosman (UCM) model to understand the relation between the physical structures of the infalling envelope around a protostar and ...their observational features in molecular lines, adopting L1527 as an example. We also compared the physical structure and synthetic position–velocity (
P–V
) diagrams of the UCM model and a simple ballistic (SB) model. There are multiple ways to compare synthetic data with observational data. We first calculated the correlation coefficient. The UCM model and the SB model show similarly good correlation with the observational data. While the correlation reflects the overall similarity between the cube datasets, we can alternatively compare specific local features, such as the centrifugal barrier in the SB model or the centrifugal radius in the UCM model. We evaluated systematic uncertainties in these methods. In the case of L1527, the stellar mass values estimated using these methods are all lower than the value derived from previous Keplerian analysis of the disk. This may indicate that the gas infall motion in the envelope is retarded by, e.g., magnetic fields. We also showed analytically that, in the UCM model, the spin-up feature of the
P–V
diagram is due to the infall velocity rather than the rotation. The line-of-sight velocity
V
is thus ∝
x
−0.5
, where
x
is the offset. If the infall is retarded, rotational velocity should dominate so that
V
is proportional to
x
−1
, as is often observed in the protostellar envelope.
Enhancement of the critical heat flux in pool boiling by the attachment of a honeycomb-structured porous plate on a heated surface is investigated experimentally using water under saturated boiling ...conditions. As the height of the honeycomb porous plate on the heated surface decreases, the CHF increases to 2.5
MW/m
2, which is approximately 2.5 times that of a plain surface (1.0
MW/m
2). Automatic liquid supply due to capillary action and reduction of the flow resistance for vapor escape due to the separation of liquid and vapor flow paths by the honeycomb-structure are verified to play an important role in the enhancement of the CHF. A simplified one-dimensional model for the capillary suction limit, in which the pressure drops due to liquid and vapor flow in the honeycomb porous plate balances the capillary force, is applied to predict the CHF. The calculated results are compared with the measured results.
Various surface modifications of the boiling surface, e.g., integrated surface structures, such as channels and micro-pin fins, and the coating of a micro-porous layer using sintered metal powders ...and nanoparticle deposition onto the heat transfer surface, have been proven to effectively enhance the critical heat flux (CHF) in saturated pool boiling. In particular, novel methods involving nanofluids have gained a great deal of attention because the CHF for the use of nano-fluids is increased drastically, by up to approximately three times compared to that of pure water. CHF enhancement using nanofluids is related to surface wettability, surface roughness, and capillary wicking performance due to nanoparticle deposition on the heated surface. Several studies have proposed the use of nanofluids to enhance the in-vessel retention (IVR) capability in the severe accident management strategy implemented at certain light-water reactors. Systems using nanofluids for IVR must be applicable to large-scale systems, i.e., sufficiently large heated surfaces compared to the characteristic length of boiling (capillary length). However, as for the effect of the size of heater with nanoparticle deposition, it was revealed that the CHF tends to be decreased with the increased heater size. On the other hand, the CHF in saturated pool boiling of water using a honeycomb porous plate was shown experimentally to become approximately twice that of a plain surface with a heated surface diameter of 30mm, which is comparatively large. The enhancement is considered to result from the capillary supply of liquid onto the heated surface through the microstructure and the release of vapor generated through the channels.
In the present paper, in order to enhance the CHF on a large heated surface, the effects of a honeycomb porous plate and/or nanoparticle deposited heat transfer surface on the CHF were investigated experimentally. As a result, the CHF was enhanced greatly by the attachment of a honeycomb porous plate to the modified heated surface by nanoparticle deposition, even in the case of a large heated surface. Under the best performing surface modifications, the CHF for 10-mm-, 30-mm- and 50-mm-diameter surfaces was enhanced up to 3.1, 2.3, and 2.2MW/m2, respectively.
•CHF is enhanced by a nanofluid and spherical porous bodies.•The proposed method can improve CHF of heated surface with curvature.•Dryout was restricted by spherical porous body in a saturated ...boiling pool of nanofluid.•Liquid was supplied by spherical porous bodies to the nanoparticle-deposited layer.
One strategy to address severe nuclear accidents is the in-vessel retention (IVR) of corium debris. IVR consists of the external cooling of the reactor vessel to remove the decay heat from the molten core through the lower head of the vessel. However, heat removal is limited by the occurrence of the critical heat flux (CHF) condition at the outer surface of the reactor vessel. Therefore, we propose a CHF enhancement technique in a saturated pool boiling by the attachment of a honeycomb porous plate (HPP) on the heated surface. However, the reactor vessel on which to install the HPP exhibits curvature, so the key to realizing IVR depends on the placement of the HPP on the curved surface of the reactor vessel. Accordingly, we propose an approach using porous cellulose beads and a nanofluid. Consequently, for the combination of the nanofluid (TiO2, 0.1 vol%) and spherical porous bodies, the CHF is demonstrated to be enhanced by up to a maximum factor of two compared to that of a plain surface of distilled water.
•CHF can be enhanced by a two-layer structured Honeycomb Porous Plate.•A small gap between the HPP and the heated surface is important for CHF enhancement.
In the present paper, we propose a novel ...critical heat flux (CHF) enhancement technique using a two-layer structured honeycomb porous plate (HPP) that can be applied in principle regardless of heater orientation. In a previous study, the CHF during saturated pool boiling of water was investigated experimentally using an HPP attached to a heated surface and was shown to be enhanced to more than twice (2.0 MW/m2) that for a plain surface. According to the proposed capillary limit model, the CHF can be increased by decreasing the thickness of the HPP because of the decrease in the frictional pressure drops caused by the liquid flow in the porous medium. However, the CHF could not be greatly enhanced when the thickness of the HPP was comparable to the thickness (approximately 100 μm) of the thin liquid film (the macro-layer thickness) formed beneath coalescent vapor bubbles. Based on the observation of the boiling configuration near the CHF, a large coalesced bubble forms on the heated surface and departs periodically. Therefore, the CHF may occur when water contained in a porous material disappears due to evaporation during the bubble hovering period. In order to prevent the dry-out phenomenon during the hovering period of a large coalesced bubble and enhance the CHF, we herein propose that the structure of HPPs should be improved by the superposition of two kinds of HPPs and that each of the HPPs must satisfy two conditions. First, an HPP simply attached to a heated surface should have very fine pores to supply water to the heated surface due to strong capillary action, and the HPPs should be as thin as possible in order to decrease the pressure drop caused by internal water flow. Second, the other HPP, which is stacked on top of the thin HPP, must be structured to hold a sufficient amount of water in order to prevent the inside of the HPP from drying out during the bubble hovering period over the plate. Moreover, for further CHF enhancement, we found that the gap between the HPP and the heated surface caused by the surface roughness is important because vapor escapes through this gap so that liquid is easily supplied to the heat transfer surface.
•Different-mode-interacting boiling on CHF was performed for various surface sizes.•Significant enhancement in CHF was achieved with nonuniform plate.•CHF decreased with increase in surface size and ...decrease in gap size.•Optimum material width value to surface size and gap width on CHF was existed.•To reduce CHF decrease due to surface size by material width increase was possible.
In this study, the effect of heating surface size (squares with 10–40 mm edges) on critical heat flux (CHF) when applying a novel method of different-mode-interacting boiling in narrow gaps in a water pool was investigated. Nonuniform heating plates with alternately arranged materials with high and low thermal conductances were applied under various material widths operated with different gaps. Significant enhancements in CHF were observed for the nonuniform heating surface compared to a uniform surface, along with a clear effect of the heating surface size on CHF. Furthermore, the effects of surface size, gap size, and material width on CHF were classified into two trends; decreasing the gap and increasing the surface size tended to monotonically decrease CHF, while the material width had an optimum value for maximizing CHF for each surface size and gap. Therefore, it has been shown that different-mode-interacting boiling was effective in improving CHF even when the surface size was increased.
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