Critical Heat Flux Values Research Articles

A methodology for CHF prediction in VVER rod bundles

The critical heat flux (CHF) is an important thermal–hydraulic parameter that must be determined during the design of water-cooled reactors to ensure safety of operation. In this paper, a methodology is proposed to predict the CHF in VVER11VVER is a transliterated acronym for “vodo-vodyanoi energetichesky reaktor” which literally means “water-water power reactor” (Nuclear Energy Agency, 2021). rod bundles. A whole-reactor model of VVER-1000 is implemented in RELAP-SCDAPSIM, and, based on the bundle-average approach, the parameters of the hot channel are used to assess four CHF predictors (i.e., the 2006 Groeneveld look-up table (LUT), and the Bowring, W-3, Biasi, and OKB-Gidropress correlations) upon comparison with the 2011 Bobkov LUT. A brief critique is given of the correction factors for both the Groeneveld and Bobkov LUTs. The effects of channel diameter and non-uniform axial heat flux on the predicted values of CHF in subcooled conditions are also studied. The performance and time complexity of the standard and Bourke trilinear interpolation algorithms are assessed for use with LUTs in safety codes. It has been concluded that the non-uniform axial heat flux does not affect the CHF in subcooled conditions and that the Groeneveld diameter factor exponent gives better predictability of the CHF in VVER rod bundles over the Tanase and Wong exponent correlations. Additionally, the Groeneveld LUT has been found to nearly reproduces the Bobkov LUT CHF values when used in conjunction with the Bobkov heated length factor. Values of MDNBR equal to 1.83 and 1.96 can be used as thermal limits at 12% overpower when the Bowring and W-3 correlations, respectively, are used in subcooled conditions. The standard and Bourke algorithms nearly exhibit the same average CPU time. The Bourke algorithm, however, is less affected by noise. Further research is needed to ensure the smoothness of the Bobkov LUT and to derive reactor type-dependent correction factor formulas for use with the Groeneveld LUT.

Parametric Effects on Pool Boiling Heat Transfer and Critical Heat Flux: A Critical Review

Abstract Pool boiling heat transfer offers high-performance cooling opportunities for thermal problems of electronics limited with high heat fluxes. Therefore, many researchers have been extensively studying over the last six decades. This paper presents a critical literature review of various parametric effects on pool boiling heat transfer and critical heat flux (CHF) such as pressure, subcooling, surface topography, surface orientation, working fluid, and combined effects. To achieve an optimal heat removal solution for a particular problem, each of these parameters must be understood. The governing mechanisms are discussed separately, and various options related to the selection of appropriate working fluids are highlighted. A broad summary of correlations developed until now for predicting CHF is presented with their ranges of validity. While proposed correlations for predicting CHF have been quite promising, they still have a considerable uncertainty (±25%). Finally, a correlation proposed by Professor Avram Bar-Cohen and his team (thermal management of electronics (TME) correlation) is compared with the experimental dataset published in previous studies. It shows that the uncertainty band can be further narrowed down to ±12.5% for dielectric liquids by using TME correlation. Furthermore, this correlation has been enhanced to predict CHF values underwater above 50 W/cm2 by applying a genetic algorithm, and new perspectives for possible future research activities are proposed.

Experimental and mechanical research on CHF characteristics during combined/bottom quenching in narrow rectangular channel

Critical heat flux (CHF) phenomenon plays an important role during quenching phase when loss of coolant accident (LOCA) occurs in plate-type fuel reactor core. Heat transfer capability can be greatly enhanced with larger CHF value so that the cladding integrity can be maintained during quenching process. However, the related research on transient CHF during quenching process in narrow rectangular channel is extremely limited in literature, not to speak of combined quenching. Considered that quenching CHF mechanism may be totally different from that during heating process, experimental and mechanical researches were conducted comprehensively on CHF behavior during combined/bottom quenching in this article.Conclusion can be obtained that during combined quenching, bottom injection portion plays a dominant role because major portion of top injection may be expelled due to counter current flow limitation (CCFL) in narrow rectangular channel. Besides, boundary conditions such as injection methods, inlet subcooling, pressure and heating power have strong effect on CHF behavior. Based on previous visualization results, mechanism of quenching CHF can be attributed to dryout of liquid sublayer beneath coalesced bubble, and the proposed CHF model matches well with experimental data (RMS error 22.24%). The relevant findings are important for safety operation of plate-type fuel reactor and also can be applied in other industrial scenes where quenching phenomenon are encountered.

A phenomenological investigation on near-wall bubble behavior close to CHF in flow boiling

The insight information of subcooled flow boiling is very important for the development of boiling crisis mechanistic model. In this paper, visual experiment is carried out in a vertical square channel with a heating rod with diameter of 9.5 mm. Based on the near-wall bubble behavior captured by a high-speed camera, combined with advanced image post-processing technique developed in-house, the qualitative and quantitative analysis of flow boiling behavior near the heating wall are presented when approaching the CHF (Critical Heat Flux). According to the visual experimental study, a detailed discussion on the phenomena before CHF and during the CHF is presented. With high heat flux, periodic large vapor clot passes with relatively constant frequency. Then periodic wavy vapor layer and liquid film clearly appear in the near-wall region when the working condition is extremely close to the CHF. It is found that high thermal-hydraulic parameters, including system pressure, mass flux and inlet subcooling, prevent the formation of thick vapor layer, and this is a significant precursor to trigger boiling crisis. This implies that the possibility of triggering boiling crisis is reduced due to the difficulty in forming thicker vapor layer, and lead to higher CHF value. Additionally, a prediction of wavelength between two-phase layers is proposed by combining visual observations and instability theory. The predicted results show good agreement compared with experimental data. Those results can provide predictions of key interfacial feature parameters for developing CHF mechanistic model.

Critical heat flux characteristics of flow boiling on a heater rod under inclined and rolling conditions

Offshore floating nuclear power plants have been developed to supply power to remote regions. Since they are operated in an ocean environment, the thermal–hydraulic phenomena in the reactor core may be different from those in the land-based reactors. However, the experimental study on critical heat flux (CHF) under ocean condition is limited and its mechanism is unclear. In this study, an experiment was performed to measure CHF in a single heater rod under inclined and rolling conditions. To create the required environment to simulate the conditions of inclination and rolling, a rolling platform was designed and constructed. It could provide an inclination of up to 45° and rolling of up to 45° with a minimum period of 6 s. In the CHF test loop, the experimental conditions were set up to correspond to pressurized water reactor conditions, using the fluid-to-fluid scaling method. The working fluid used was R134a. The test section was a concentric annular channel composed of a heater rod and an unheated tube housing. The result of the inclination experiment suggested that in the most cases, the CHF was enhanced when compared with the vertical condition. In a limited number of cases of low-pressure and high-flow conditions, the CHF was degraded as compared to the vertical condition. These variations were proportional to the inclination angle and inversely proportional to the mass flux. From the results of the rolling experiment, a precursor of CHF was observed with a large fluctuation in temperature before the CHF, and its trigger was related to the CHF value of the inclined case. Based on this observation, the overall relations between the vertical, inclined, and rolling CHF values were derived. It was concluded that the rolling motion weakened the CHF enhancement and degradation effects of the inclination.

Visualization experiments and a new correlation of critical heat flux in a narrow rectangular channel

Critical heat flux (CHF) is a crucial factor in ensuring safe operation and improving the economy of the plant. This study carried out a visualization experiment to identify the dominated parameters affecting the CHF in a narrow rectangular channel at different gap sizes (1–5 mm). The experiments were performed at pressures ranging from 1 to 4 MPa, with inlet subcooling ranging from 65 to 120 K and mass flux ranging from 350 to 2000 kg/(m2·s). A large amount of CHF correlations were compared with experimental values, and the results proved that among them Sudo correlation has the smallest prediction error. The Look-up Table method is suitable for predicting CHF in narrow rectangular channels, considering the limitation of the gap size by employing a correction factor. The increase of gap size, system pressure, inlet subcooling, and mass flux contributes to enhancing the CHF values. A new CHF correlation was proposed based on these parameters through step regression method. The comparison between prediction and experimental data demonstrates that the new correlation can accurately predict the CHF in a narrow rectangular channel with a maximum discrepancy of 20%.

Accurate prediction of the critical heat flux for pool boiling on the heater substrate

While the influence of liquid qualities, surface morphology, and operating circumstances on critical heat flux (CHF) in pool boiling has been extensively studied, the effect of the heater substrate has not. Based on the force balance analysis, a theoretical model has been developed to accurately predict the CHF in pool boiling on a heater substrate. An analytical expression for the CHF of a heater substrate is obtained in terms of the surface thermophysical property. It is indicated that the ratio of thermal conductivity (k) to the product of density (ρ) and specific heat (c p) is an essential substrate property that influences the CHF. By modifying the well-known force-balance-based CHF model (Kandlikar model), the thermal characteristics of the substrate are taken into consideration. The bias of predicted CHF values are within 5% compared with the experimental results.

A remarkable CHF of 345W/cm2 is achieved in a wicked-microchannel using HFE-7100

Boiling heat transfer of dielectric fluids is a promising cooling technique for thermal management of microelectronic systems. However, the critical heat flux achievable is generally low, because of the poor thermophysical properties of these fluids. To address this dilemma, we propose a new cooling concept to substantially enhance global liquid supply during phase-change process using enhanced capillary-driven force. Additionally, dedicated vapor pathways are designed among a bank of micro-pillars to facilitate vapor removal. In this work, new wicks comprised of silicon micro-pinfin arrays are explored to significantly enhance the flow boiling heat transfer performance. To examine the functionalities of this wick, experiments on HFE-7100 were carried out with mass velocities varying from 247 to 3,465 kg/m2s. To explore the enhancement mechanisms and to analyze the capillary-assisted flow boiling process, visualization studies were conducted. The results indicate that sustainable evaporation induced by wick microstructures and efficient liquid supply are the enhancement mechanisms compared to parallel microchannels with solid walls. It is found that the overall heat transfer coefficient is substantially increased up to 75%. Remarkably, a high critical heat flux (CHF) of approximately 345 W/cm2 is recorded at G = 3,465 kg/m2s at coolant inlet temperature of ∼20 °C. Equally importantly, this noticeable enhancement of CHF value is associated with drastically decreased pressure drops compared to microchannels decorated with μ-pinfin fences.

Investigation of Critical Heat Flux for Plutonium-Based Mixed Oxide Advanced Fuel Bundle Design

Abstract The critical heat flux (CHF) performance of an advanced plutonium-based mixed oxide (MOX) fuel for potential use in a pressure tube heavy water reactor (PT-HWR) has been studied experimentally at Canadian Nuclear Laboratories with an electrically heated string simulator of 43-element fuel bundles. The fuel simulator has a uniform axial power profile and a radial power profile that is representative of the plutonium-based MOX fuel. The measurements of CHF caused by liquid-film dryout were made in the MR-3 heat transfer loop facility using R-134a refrigerant as the working fluid. The test matrix included system pressures from 1.47 to 2.11 MPa, mass flow rates from 12.7 to 14.7 kg/s and inlet temperatures from 31 to 59 °C, which are representative of the water-equivalent reactor operating conditions of 9 to 12.5 MPa pressure, 13.5 to 21.3 kg/s mass flow rate and the desired inlet subcoolings. Compared to conventional natural uranium fuel, the radial power profile of a MOX fuel exhibits a steeper and less even distribution across the fuel element rings, with a relatively higher power in the outer ring. It was found that CHF values of the MOX fuel are significantly lower than those of the natural uranium fuel. Based on the experimental data, a correlation has been derived to account for the effect of radial power profile on CHF. This correlation can be used to evaluate the relative CHF values of advanced or nonconventional fuel designs with radial power profiles deviating from that of natural uranium fuel.

Experimental investigation of dryout phenomena under oscillatory flow conditions

In water-cooled nuclear reactors, the maximum power which can be extracted from the core is limited by critical heat flux (CHF). CHF in the high-quality region is known as dryout. In advanced nuclear reactors, the coolant flow occurs solely by virtue of natural circulation; however, instabilities may occur during off-normal operations. This may lead to premature dryout due to lower coolant flow rates seen by the heater during such oscillations. This paper describes the experimental investigation on the effect of flow oscillations on the CHF with the time period of 120 s, which is observed typically in the large-scale natural circulation system. Based on observations made with respect to temperature transient, the continuous dryout is preceded by the transient dryout for higher flow oscillations. But as flow fluctuation decreases, the transient dryout phenomenon is found to disappear. The applicability of the look-up table to predict CHF under oscillatory flow conditions using suitable correction factors (CFs) for premature dryout has been evaluated. CFs for the CHF under oscillations suggested by previous authors have been compared. The maximum possible degradation in CHF value suggested by previous authors has been found to agree with the present experimental data. Percentage fluctuation in heat transfer coefficient (HTC) at fully developed annular flow conditions has been evaluated, and it is found that fluctuation in HTC is in phase with the fluctuation in flow.

CFD investigation of the cold wall effect on CHF in a 5 × 5 rod bundle for PWRs

In this study, a set of mathematic models suitable for flow boiling simulation and CHF prediction in rod bundle channels of Pressurized Water Reactor (PWR), including Eulerian two-phase model combined with wall boiling model, were established using CFD method. The models were validated against the international flow boiling and Critical Heat Flux (CHF) experiment benchmark. The results show that the error between the calculated CHF and the experimental data is less than 10%. Then a 3-D model of 5 × 5 rod bundles with cold wall was built and the detailed three-dimensional thermal–hydraulic characteristics were investigated, especially emphasising on the cold wall effect. The existance of cold wall would affect the coolant thermal mixing intensity, leading to a certain degree influence of the CHF. Results show that under the same inlet conditions, the calculated CHF value after introducing the cold wall is higher by 11.4%, while the overall power decreases due to the existence of cold wall. Besides, the pressure drop and temperature in the channel would decrease, while the distribution of void fraction basically remain unchanged, which is still mainly affected by the spacer grids and mixing vanes. Compared with the modified W-3 formula, the error of the CHF value calculated by CFD method is less than 10%, and the CFD method can show the three-dimensional distribution characteristics and evolution process of physical parameters.

Fluid-to-Fluid modelling of CHF at High-Pressure subcooled water conditions

An experiment was carried out to obtain data on critical heat flux (CHF) in an 8-mm vertical heated tube cooled with Refrigerant (R-134a) at high-pressure subcooled and low-quality conditions. Parametric trends of the CHF data were examined and have been shown consistent with those obtained in experiments with water flow. Variations of CHF depends mainly on local flow conditions and the length effect appears to be diminished at length-to-diameter (L/D) ratios from 30 to 200. The Katto scaling law has been shown applicable for transforming R-134a CHF values into water CHF values at subcooled and low-quality conditions. The R-134a CHF data have been transformed using the Katto fluid-to-fluid scaling law and assessed against the water CHF values obtained from the 2006 CHF look-up table (LUT-2006). The results showed that the LUT-2006 is conservative and need further development.

Hydrophilic and Hydrophobic Nanostructured Copper Surfaces for Efficient Pool Boiling Heat Transfer with Water, Water/Butanol Mixtures and Novec 649.

Increasing heat dissipation requirements of small and miniature devices demands advanced cooling methods, such as application of immersion cooling via boiling heat transfer. In this study, functionalized copper surfaces for enhanced heat transfer are developed and evaluated. Samples are functionalized using a chemical oxidation treatment with subsequent hydrophobization of selected surfaces with a fluorinated silane. Pool boiling tests with water, water/1-butanol mixture with self-rewetting properties and a novel dielectric fluid with low GWP (Novec™ 649) are conducted to evaluate the boiling performance of individual surfaces. The results show that hydrophobized functionalized surfaces covered by microcavities with diameters between 40 nm and 2 µm exhibit increased heat transfer coefficient (HTC; enhancements up to 120%) and critical heat flux (CHF; enhancements up to 64%) values in comparison with the untreated reference surface, complemented by favorable fabrication repeatability. Positive surface stability is observed in contact with water, while both the self-rewetting fluids and Novec™ 649 gradually degrade the boiling performance and in some cases also the surface itself. The use of water/1-butanol mixtures in particular results in surface chemistry and morphology changes, as observed using SEM imaging and Raman spectroscopy. This seems to be neglected in the available literature and should be focused on in further studies.

Ultra-Compact Microscale Heat Exchanger for Advanced Thermal Management in Data Centers

Abstract This study is focused on the experimental characterization of two-phase heat transfer performance and pressure drops within an ultracompact heat exchanger (UCHE) suitable for electronics cooling applications. In this specific work, the UCHE prototype is anticipated to be a critical component for realizing a new passive two-phase cooling technology for high-power server racks, as it is more compact and lighter weight than conventional heat exchangers. This technology makes use of a novel combination of thermosyphon loops, at the server-level and rack-level, to passively cool an entire rack. In the proposed two-phase cooling technology, a smaller form factor UCHE is used to transfer heat from the server-level thermosyphon cooling loop to the rack-level thermosyphon cooling loop, while a larger form factor UCHE is used to reject the total heat from the server rack into the facility-level cooling loop. The UCHE is composed of a double-side-copper finned plate enclosed in a stainless steel enclosure. The geometry of the fins and channels on both sides are optimized to enhance the heat transfer performance and flow stability while minimizing the pressure drops. These features make the UCHE the ideal component for thermosyphon cooling systems, where low pressure drops are required to achieve high passive flow circulation rates and thus achieve high critical heat flux values. The UCHE's thermal-hydraulic performance is first evaluated in a pump-driven system at the former laboratory of heat and mass transfer (LTCM)-Swiss Federal Institute of Technology of Lausanne, where experiments include many configurations and operating conditions. Then, the UCHE is installed and tested as the condenser of a thermosyphon loop that rejects heat to a pumped refrigerant system at Nokia Bell Labs, in which both sides operate with refrigerants in phase change (condensation-to-boiling). Experimental results demonstrate high thermal performance with a maximum heat dissipation density of 5455 (kW/m3/K), which is significantly larger than conventional air-cooled heat exchangers and liquid-cooled small pressing depth brazed plate heat exchangers. Finally, a thermal performance analysis is presented that provides guidelines in terms of heat density dissipations at the server- and rack-level when using passive two-phase cooling.

Unified descriptor for enhanced critical heat flux during pool boiling of hemi-wicking surfaces

Boiling heat transfer is dictated by interfacial phenomena at the three-phase contact line where vapor bubbles form on the surface. Structured surfaces have shown significant enhancement in critical heat flux (CHF) during pool boiling by tailoring interfacial phenomena. This CHF enhancement has been primarily explained by two structural effects: roughness, which extends the contact line length at the bubble base, and wickability, the ability to imbibe liquid through surface structures by capillary pumping. In this work, we show that CHF enhancement on structured surfaces cannot be described by roughness or wickability alone. This result was confirmed using systematically designed micropillar surfaces with controlled roughness and wickability. Further, we performed a scaling analysis and derived a unified descriptor, which represents the combined effects of thin film density and volumetric wicking rate. This unified descriptor shows a reasonable correlation with CHF values with our experiments and literature data. This work provides important insights in understanding the role of surface structures on CHF enhancement, thereby providing guidelines for the systematic design of surface structures for enhanced pool boiling heat transfer.

Enhancement of critical heat flux (CHF) in pool boiling with anodized copper surfaces

Present work reports the creation of Cu(OH)2 nanoneedles on a copper surface using an anodization process for extending critical heat flux (CHF) limit in pool boiling applications. Three anodized samples have been prepared by applying three different current densities. Surface morphology and wetting characteristics of the anodized surfaces are presented. The characteristics of nanoneedles depend upon the applied current density. The formation of Cu(OH)2 nanoneedles during the anodization process is confirmed by phase analysis using XRD confirms. The formation of Cu(OH)2 nanoneedles increases the wettability and wickability of the anodized surfaces compared to the plain surface. At high current density, the anodized surfaces show nearly super hydrophilic behaviour. Due to enhanced wettability, the wall superheat required for nucleation is more, and nucleation site density is less for anodized copper surfaces. Bubble departure size is also more for anodized surfaces compared to the plain copper surface. The anodized surfaces show a higher CHF value compared to the plain copper surface. Rewetting of the boiling surface occurs by capillary wicking in the anodized surfaces, which delays the dryout phenomena resulting in CHF enhancement. A maximum enhancement of 39.74% in CHF is observed for the C-9 anodized surface compared to the plain copper surface.

Experimental parametric study of hierarchical micro/nano electrodeposited (six-step) pattern with respect to volcano-shape morphology in pool boiling performance augmentation

ABSTRACT One of the methods to improve the pool boiling process is electrodeposition, which is cheap, simple, and creates a strong bond in the structure. Besides, it can produce controllable porosity and improve boiling performance. In this paper, experiments were conducted in atmospheric conditions, using deionized water and hierarchical patterns created by a combination of 6-step electrodeposition and photolithography. The modified surfaces are characterized by X-ray diffraction (XRD), scanning electronic microscopy (SEM) images, and profilometric analysis. Besides, the effect of various parameters, including diameter, distance from perimeter to perimeter and wettability, along with the effect of nucleation site’s density (NSD), and effective surface area were investigated. Prediction of critical heat flux (CHF) values and experimental data showed that capillary effect of these surfaces increased the maximum heat flux compared to the plain surface. Also, changing the morphology of deposition and creating the volcano-shape structures were identified as a more suitable place for nucleation and enhancing the capillary effect and the effect of porosity and roughness. The maximum heat flux and heat transfer coefficient (HTC) for sample number 16 reached 145.36 and 13.13 respectively, which was proportional to the improvement of 118.8% and 119% relative to the plain surface.

Bench-scale experimental study on the fire behavior of electric cable arrays by considering different layouts

The effect of different cable layouts on the fire behavior of electric cable arrays was experimentally studied. The influence of external heat flux on cable fire characteristics was investigated. Several parameters for electrical cables such as the post-burning morphology, ignition time, heat release rate, peak heat release rate and total heat release were obtained. The results show that cable layouts could affect cable charring degrees according to the post-burning morphology. A linear relationship was found in the transformed form of time to ignition and radiant heat flux, and the critical radiant heat flux value for the single cable array appeared smaller than that for the other two layouts. The peak heat release rate for Cables A–D with the single array presents the increasing trend with an increase in radiant heat flux, while the two parallel and intersectional cable arrays present the different trends. Moreover, the total heat release values of Cables A–D in the different cable layouts were analyzed. This work provides the basic data and preliminary investigation to fire engineering of cable arrays with the different layouts.

Experimental investigation of the influence of boiling-induced ageing on high heat flux flow boiling in a copper microchannel

Copper, due to its ease of machining and high thermal conductivity, is the most commonly used substrate for heat sink applications. This paper presents an experimental investigation of the effect of boiling-induced copper ageing on the heat transfer and critical heat flux (CHF) characteristics of flow boiling of water in a mini/micro-channel. Ageing study was conducted for two cases; Case-1 consists of repeated boiling runs in a channel with freshly machined copper surface for a constant mass flux, and Case-2 consists of repeated boiling runs in the same channel with the copper surface obtained after cleaning the aged surface with 0.1 M HCl solution. A unique feature of the test section design is that the microchannel, which is 0.47 mm deep, 1.01 mm wide and 40 mm long, has been machined on the PEEK cover plate. This enables the in-situ measurement of contact angle on the channel copper surface, which is heated and undergoes ageing during the course of the experimental runs. The heat transfer coefficient reduces by up to 63% and 70% for Case-1 and Case-2, respectively. However, the results of both Case-1 and Case-2 indicate no significant change in the CHF due to repeated boiling runs. The EDS analysis of the channel copper surface indicates thermal oxidation during ageing, and the SEM and confocal microscopy analyses show the change in surface morphology due to ageing. The results of the contact angle measurement indicate that the copper surface becomes hydrophilic after ageing. The average static contact angle on the surface reduced from 91ο to 32ο during the Case-1 study and from 98ο to 28ο during the Case-2 study. The possible reasons for the reduction in the boiling heat transfer coefficient and insignificant change in the CHF are discussed. Experiments were also carried out to study the effect of mass flux on the CHF using the fully aged channel. The CHF was found to increase with the increase in mass flux. The study compares the obtained experimental CHF values with the predictions made by the correlations available in literature.

CHF enhancement of downward-facing saturated pool boiling on the SCGS-modified surfaces with multi-scale conical pin fin structures

Enhancing the critical heat flux (CHF) of external reactor vessel cooling (ERVC) to achieve the strategy of in-vessel retention (IVR) has been deemed as an essential requirement for the security of modern commercial PWR nuclear power plants. A new type of multi-scale conical pin fin structures had been designed to meet this need. These conical structures of four different axial heights (1.0, 1.5, 2.0 & 2.5mm) and similar radial geometric parameters were fabricated on four heating surfaces of copper substrates by supersonic cold gas spray technique (SCGS), and the conical pin fin structures were combined with a thin layer of micro porous structures. Transient-state (quenching) and steady-state (heating) pool boiling experiments had been respectively conducted on 6 different downward-facing surfaces under different inclination angles including 5°, 30°, 45°, 60°, 75° & 90° (in vertical), which included above-mentioned four pin fin surfaces, a plain surface and a coated surface covered with a 1mm-thick layer of uniform porous coating fabricated by SCGS with copper particles. The experimental results showed that the CHF increased with the inclination angles. The CHF of all the SCGS-modified surfaces were significantly enhanced compared with those of the plain surface. The maximum CHF value and the maximum CHF enhancement belonged to the 2.0mm-height pin fin surface whose CHF values were more than 1.7 times of those of the plain surface, and up to 2.1 times in 90°. The 1.0mm pin fin surface had the greatest performance in boiling heat transfer coefficient (HTC) and its CHF-enhancing performance also greatly exceeded that of 1.0mm uniform porous coating surface. Remarkable performances of multi-scale conical pin fin structures had been experimentally confirmed, hence the safety margin of IVR-ERVC will be greatly enhanced after adopting them. In addition, the CHF enhancement mechanism of the multi-scale conical pin fin structures was analyzed.