Radial Heat Losses Research Articles

The effect of the radial heat loss, upper reservoir temperature, and corrugated wall channel on the flooding critical heat flux

This study measured the flooding critical heat flux (flooding CHF) for two vertical annular channels with different channel gaps. The experiments were conducted at atmospheric pressure and with no flow. The effects of the upper reservoir temperature within 30–80 °C and radial heat loss have been studied. It was found that the upper reservoir temperature has a small effect on the flooding CHF values, and it varied in the range of up to 10% compared to the saturated conditions. On the other hand, a significant effect of the radial heat loss was observed on the measured flooding CHF values (up to 100%). The flooding CHF experimental results were compared with numerous empirical correlations from the literature, some initially based on air-water counter-current flow experiments (flooding experiments with no heating). It was found that based on the onset flooding velocity experiments, the flooding CHF results can be successfully predicted. The effect of the wall corrugation on the flooding CHF was also examined in the simple air-water counter-current flow setup. This study shows that the air flooding velocity in a corrugated channel is higher than in a smooth channel with the same cross-section geometry and dimensions. Therefore, using a smooth channel flooding CHF value for a corrugated one guarantees a conservative result.

Evaluation of technical and economic potential of waste heat distribution networks in industrial sites

This paper presents a methodology for the evaluation of waste heat distribution networks in industrial sites. In the context of ultra-efficient industrial sites, implementing these networks shows significant energy cost and emission reduction potential through cooperation between companies. To this end, a mathematical methodology for waste heat distribution network simulation was developed. In this approach, a differential equation relating radial heat losses and internal temperature was derived to include temperature changes inside the pipe system. Furthermore, relevant parameters such as diameter, insulation thickness, and temperature at source and sink were used to determine the distribution network’s technical and economic feasibility . Using time discrete, characteristic thermal energy supply and demand data in the analysis framework, the technical and economic assessment of waste heat distribution networks was carried out via the use case of an industrial park. In this use case, the economic and ecological advantages were quantified and shown.

Modeling smoldering ignition by an irradiation spot

Irradiation spots, such as lasers, lightning strikes, and concentrated sunlight, are common ignition sources in building and wildland fires, where smoldering is generally first ignited and then transitions to flaming. In this work, a physics-based 2-D computational model that integrates heat-and-mass transfer and heterogeneous chemistry is built to investigate the smoldering ignition of typical solid fuels using irradiation spots. Simulation results predict that, given the size of the irradiation spot, the ignition time decreases as the radiant heat flux increases. However, as the diameter of the irradiation spot decreases, the modeled minimum heat flux of smoldering ignition increases significantly, agreeing well with experiments and theoretical analysis. When the irradiation spot is smaller than 20-50 mm, assumptions of constant ignition temperature and fuel-burning flux become invalid. The commonly-used physical dimensions of thermally thin/thick fuels are not applicable for smoldering spotting ignition due to the significant radial conductive heat loss in the lateral direction. Further analyses show that the minimum irradiation of smoldering ignition increases as the fuel thickness increases, but it is insensitive to the fuel moisture content. This is the first time that a sophisticated 2-D model has been used to predict the smoldering ignition using irradiation spots, which deepens the understanding of the ignition by a remote heating source and large irradiation.

Investigation of multi-dimensional transfer effects in applied smouldering systems: A 2D numerical modelling approach

Applied smouldering systems are gaining popularity for a variety of energy conversion applications. Radial heat loss plays a crucial role in these systems, as they cause multi-dimensional effects (e.g., in temperature, airflow, and chemical activity). These effects can control system operation limits and performance; therefore, a robust understanding of these multi-dimensional effects is crucial for design engineers. A multi-dimensional applied smouldering numerical model was developed that couples key physics and chemistry. The model was validated against highly instrumented smouldering experiments. The model was then used to qualitatively investigate the multi-dimensional effects and quantitatively analyze the energy balance that dictates the limits of the self-sustaining process. Moreover, a sensitivity analysis of the system energy efficiency, air flow, fuel concentration, and porous medium permeability was completed. The results provide insight into the interconnected nature of key physical (e.g., temperature, air flow, permeability) and chemical (e.g., oxygen concentration, reaction intensity) qualities. Altogether, this work provides a novel tool for investigating, designing, and optimizing smouldering reactors for a range of applications such as soil remediation, waste-to-energy, and improving sanitation in the developing world.

The improved energy efficiency of applied smouldering systems with increasing scale

Smouldering combustion has been demonstrated to be a highly energy efficient approach towards waste-to-energy. The benefits of smouldering are principally due to the matching of the energy generation and transfer time scales as well as its low quenching temperature (< 400 °C). This enables effective energy extraction of problematic wastes (e.g., because of low-volatility or high-moisture content). As the engineering applications of smouldering combustion expand, there is a growing interest in designing systems to best house a propagating smouldering reaction. Through a series of experiments, this work quantified the role of radial heat losses in the energy efficiency of different sized reactors. Thermocouples were placed throughout radial and axial coordinates and integrated to estimate the net stored energy throughout the column volume with time. The impact of heat losses was normalized as the system energy efficiency, by dividing the net stored energy by the energy added into the column for ignition and released from smouldering. The results revealed that the system energy efficiency increased from 65 ± 3% to 86 ± 5% with column radius increasing from 0.080 m to 0.300 m, respectively. As a result, scenarios that were not self-sustaining in the thin column were demonstrated to be self-sustaining in the wider column. Thus, increased system energy efficiency increased the robustness of the reaction to quenching. Altogether, this work underscores the importance of scale as a crucial design parameter enabling a smouldering system to be used as an effective waste-to-energy approach.

Heat transfer performance investigation on a finned tube adsorbent bed with a compound parabolic concentrator (CPC) for solar adsorption refrigeration

In this work, activated carbon-methanol was used as the working pair in a solar adsorption refrigeration system (SAR). The thermal conductivity of the activated carbon was very poor, resulting in performance delays of the system. To improve the performance of the heat transfer for the adsorbent bed, a new adsorbent bed with finned tubes was designed and studied using commercial computational fluid dynamics (CFD) software. Two-dimensional (2D) numerical models of two kinds of adsorbent tubes (finned and smooth tubes) were constructed and simulated. The two different 2D numerical models had similar cell numbers and the same boundaries and initial conditions. An experiment for a grid verification model of the designed finned tube was conducted, and the model of the finned tube was validated via comparison between the numerical and experimental results. In addition, the heat performances of the newly designed finned tube and the smooth tube were compared in this paper. The temperature gradients of the activated carbon in the smooth and finned tubes were approximately 28.1 °C and 4 °C during isosteric heating, respectively. The conclusions are that the fins had a large effect on the thermal performance of SAR, and the possibility that the methanol was adsorbed again by the activated carbon of the lower temperature part in the finned tubes during isobaric heating was decreased greatly. Moreover, the radial heat loss of the finned tube wall was also less than the smooth tube, which may be an important factor for improving the performance of the system effectively.

Numerical analysis for electromagnetic field influence on heat transfer behaviors in cold crucible used for directional solidification

Cold crucible directional solidification (CCDS) is the technique that the materials in the segmented water-cooled crucible are induction heated and heat transfer controlled by electromagnetic field for directional solidifying crystals without contamination. In order to control crystal growth by the CCDS efficiently, mathematical analysis of heat transfer during the CCDS process is presented by means of a combined electromagnetic, fluid and thermal model. A 3-D model is established and verified to investigate the characteristics of heat transfer in the melt, which considers the coupled effects of the shaped melt and the electromagnetically driven flow on the temperature field and the shape of solid–liquid (S/L) interface. The shaped melt by electromagnetic pressure along with the induction heat in the skin layer reduce and compensate the radial heat loss, which could decrease the radial temperature gradient in the melt with increasing input powers. A vigorous three-dimensional melt flow consisting of meridional flow and azimuthal flow is induced in the melt, which improves the uniformity of the temperature field and decreases the deflection of S/L interface in the center of melt. The solidification front changes from ‘W’ shape to planar when the power reaches 39 kW. Based on the above results and analysis, TiAl alloys are successfully directionally solidified by the CCDS method with planar S/L interface, and the growth direction of columnar crystals are nearly paralleled.

3D modeling of a CPOX-reformer including detailed chemistry and radiation effects with DUO

The impact of radiation heat transfer and radial heat losses in small-scale monoliths, as often used for testing catalysts or qualifying process conditions, are an important consideration to design and predict performance of commercial size reactors. The paper presents the 3D modeling of a honeycomb CPOX (Catalytic Partial Oxidation) reformer, including detailed surface chemistry for the conversion of methane on rhodium. The calculation domain comprises the flow region and two monoliths (one of them coated) which are positioned in a glass tube. For the simulations the software tool DUO (coupling between OpenFOAM and DETCHEM™) was used. The objective was to model the system without any boundary conditions for the temperature (aside from the inlet). As the temperature level is above 900K solid body radiation has to be included. The comparison of the results with detailed experimental data shows that it is possible to reproduce the species concentrations and the temperature fields of the flow and solid structures well. The effect of radiation, leading to a heat transfer between the two monoliths, can clearly be indicated. However, this effect plays only a minor role with respect to the chemical conversion. The simulations capture the measured effect of radial heat removal on the conversion process in different channels inside the catalyst.

Interpretation and Analysis of Transient-Sandface- and Wellbore-Temperature Data

Summary This paper presents new analytical and semianalytical solutions derived from a coupled transient-wellbore/reservoir thermal model to investigate the information content of transient-temperature measurement made within the vertical wellbore across from the producing horizon or at a gauge depth above it during drawdown and buildup tests. The solutions consider flow of a slightly compressible, single-phase fluid in a homogeneous infinite-acting reservoir system with skin modeled as a composite zone adjacent to the wellbore and account for the Joule-Thomson (J-T) heating/cooling, adiabatic-fluid expansion, conduction and convection effects both in the wellbore and reservoir. They are developed depending on the assumption that the effects of temperature changes on wellbore and reservoir-pressure-transient data can be neglected so that the mass-, momentum-, and energy-balance equations in the wellbore and reservoir can be decoupled. The semianalytical solution for predicting sandface temperatures is verified by use of a general-purpose thermal simulator. Wellbore temperatures at a certain gauge depth are evaluated through the analytical steady-state and transient-wellbore-temperature equations coupled with a semianalytical reservoir-temperature model accounting for conservation of momentum in the wellbore. Results show that drawdown- and buildup-sandface-temperature data may exhibit two semilog straight lines: one at early times reflecting the effects of adiabatic-fluid expansion in the skin zone near the wellbore, and the other, the late-time semilog straight line, reflecting the J-T effects and exhibiting the nonskin-zone properties. However, the wellbore-temperature measurements made at locations above the producing horizon may not exhibit these semilog straight lines because they are strongly dependent upon distance above the producing horizon, geothermal gradient, and radial-heat losses from the wellbore fluid to the formation on the way to gauge. It is found that the skin-zone properties are very difficult to be estimated from drawdown- and buildup-wellbore temperatures unless the gauge location is not far from the producing zone. Specifically, we found that buildup-wellbore temperature is mostly dominated by wellbore-heat losses compared with drawdown-wellbore-temperature data, and hence may not be useful to estimate the formation properties, including skin-zone properties.

Universal Scaling of Robust Thermal Hot Spot and Ionic Current Enhancement by Focused Ohmic Heating in a Conic Nanopore.

A stable nanoscale thermal hot spot, with temperature approaching 100 °C, is shown to be sustained by localized Ohmic heating of a focused electric field at the tip of a slender conic nanopore. The self-similar (length-independent) conic geometry allows us to match the singular heat source at the tip to the singular radial heat loss from the slender cone to obtain a self-similar steady temperature profile along the cone and the resulting ionic current conductance enhancement due to viscosity reduction. The universal scaling, which depends only on a single dimensionless parameter Z, collapses the measured conductance data and computed temperature profiles in ion-track conic nanopores and conic nanopipettes. The collapsed numerical data reveal universal values for the hot-spot location and temperature in an aqueous electrolyte.

Numerical simulation of a structured catalytic methane reformer by DUO: The new computational interface for OpenFOAM® and DETCHEM™

The catalytic partial oxidation of methane over a honeycomb-structured Rh/Al2O3 coated catalyst is studied at a molar C/O ratio of unity and short-contact times experimentally and numerically. Axial species and temperature profiles inside the catalytic monolith are measured by a capillary sampling technique. A detailed numerical analysis of the flow, temperature and species concentration profiles is conducted using the new software tool DUO, which is an interface for the coupling of the computational tools OpenFOAM® and DETCHEM™. This interface enables the integration of detailed surface chemistry into OpenFOAM® and allows the time-efficient calculation of structured catalysts and other catalytic reactors with multiple combined fluid and solid regions including heterogeneous reactions. In comparison with experimental data, spatial profiles of species and temperature are analyzed. The thermal boundary conditions are shown to have a strong impact on reliable predictions. Even though the characteristics of the influence of radial heat loss on the conversion can be governed, the use of a radiation model is recommended.

An improved steady-state apparatus for measuring thermal conductivity of soils

The thermal conductivity of soil is among the most critical parameters required to design ground heat exchangers, which are widely used as a renewable technology for providing heating and cooling for buildings. This paper describes the development of new test apparatus that can be used for soil specimens obtained from routine site investigation as well as reconstituted specimens. The design of the apparatus is based on the application of Fourier’s law where one directional uniform heat flux is generated through two identical specimens, producing a measurable temperature gradient that is used to calculate the thermal conductivity of the specimen. A new concept of minimizing the radial heat losses using a thermal jacket as a heat insulation barrier was examined. It was found that the no-radial heat losses condition can be achieved with thermal jacket temperature approximately equal to the average value of the ambient temperature and average specimen temperature. All parameters that can affect the measurements have been tested and the results showed a good performance with margin of error upto 5%. An application of the new test procedures involved conducting several experimental tests on undisturbed and reconstituted soil samples highlighted the simplicity of this apparatus in measuring thermal conductivity of soil under different conditions.

Ti/C‐3Ni/Al as a Replacement Time Delay Composition

AbstractReplacement reactive systems for the tungsten delay composition (W/BaCrO4/KClO4/diatomaceous earth) are needed due to recent concerns over the toxicity of hexavalent chromium and perchlorates. Systems based on condensed phase reactions, that are typically used in combustion synthesis (e.g., Ti/C or Ni/Al) are of interest as replacements due to their wide range of combustion velocities and potentially low environmental impact. In this work, the combustion characteristics of the Ti/C‐3Ni/Al reactive system were examined in microchannels with inner diameters ranging from 3.0–6.0 mm (i.e., similar to that of a common delay housing). It was found that this reactive system could be tailored to overcome the heat losses associated with small diameter microchannels by changing the relative amounts of Ti/C and 3Ni/Al. At 40 wt.‐% Ti/C content, the failure diameter was found to be between 3.0 and 4.0 mm, while at 30 wt.‐% Ti/C the failure diameter was between 4.8 and 6.0 mm. Measured combustion temperatures in metal microchannels were approximately 1700 K while those of unconfined pellets were around 100 K greater. Increasing Ti/C content resulted in faster combustion velocities while decreasing microchannel diameter resulted in slower combustion velocities. At these small sizes the effects of adding a thermal barrier (specifically Grafoil™) to minimize radial heat losses to the microchannel were shown to be minimal with respect to combustion velocity. The Ti/C‐3Ni/Al system was shown to be a suitable delay fuze composition with tunable combustion velocities ranging from 2.1–38.1 mm s−1 in aluminum microchannels with diameters ranging from 4.0–6.0 mm.

Catalytic partial oxidation of ethanol over Rh/Al2O3: Spatially resolved temperature and concentration profiles

The catalytic partial oxidation (CPOX) of ethanol was studied over rhodium/alumina-coated monoliths. An in situ sampling technique was used for ethanol CPOX, yielding spatially resolved temperature and concentration profiles. Molar C/O ratios from 0.65 to 0.85 were studied. Catalytic and blank experiments were both performed. The investigations clearly showed two zones inside the catalytic channel, an oxy-reforming zone as long as oxygen is present, with total oxidation and steam reforming as prevalent reactions, and a reforming zone, with steam reforming as the dominating reaction. Moreover, homogeneous gas-phase reactions, leading mainly to acetaldehyde and water, were observed in front of the catalyst. This was confirmed by blank experiments, which showed that homogeneous conversion of ethanol by oxidative dehydrogenation may occur in absence of a catalyst at temperatures higher than 640K. Additionally, axial profiles were collected at a fixed C/O ratio in three catalytic channels, which were differently located across the monolith. A radial heat loss from the center to the outer channels of the monolith was observed, which led to variations in the axial concentration profiles across the honeycomb.

Role of the outer-edge flame on flame extinction in nitrogen-diluted non-premixed counterflow flames with finite burner diameters

This study of nitrogen-diluted non-premixed counterflow flames with finite burner diameters investigates the important role of the outer-edge flame on flame extinction through experimental and numerical analyses. It explores flame stability diagrams mapping the flame extinction response of nitrogen-diluted non-premixed counterflow flames to varying global strain rates in terms of burner diameter, burner gap, and velocity ratio. A critical nitrogen mole fraction exists beyond which the flame cannot be sustained; the critical nitrogen mole fraction versus global strain rate curves have C-shapes for various burner diameters, burner gaps, and velocity ratios. At sufficiently high strain-rate flames, these curves collapse into one curve; therefore, the flames follow the one-dimensional flame response of a typical diffusion flame. Low strain-rate flames are significantly affected by radial conductive heat loss, and therefore flame length.Three flame extinction modes are identified: flame extinction through shrinkage of the outer-edge flame with or without oscillations at the outer-edge flame prior to the extinction, and flame extinction through a flame hole at the flame center. The extinction modes are significantly affected by the behavior of the outer-edge flame. Detailed explanations are provided based on the measured flame-surface temperature and numerical evaluation of the fractional contribution of each term in the energy equation. Radial conductive heat loss at the flame edge to ambience is the main mechanism of extinction through shrinkage of the outer-edge flame in low strain-rate flames. Reduction of the burner diameter can extend the flame extinction mode by shrinking the outer-edge flame in higher strain-rate flames.

Comment on “Evaporation from soils under thermal boundary conditions: Experimental and modeling investigation to compare equilibrium and nonequilibrium based approaches,” by Kathleen M. Smits, Abdullah Cihan, Toshihiro Sakaki, and Tissa H. Illangasekare

Comment on “Evaporation from soils under thermal boundary conditions: Experimental and modeling investigation to compare equilibrium and nonequilibrium based approaches,” by Kathleen M. Smits, Abdullah Cihan, Toshihiro Sakaki, and Tissa H. Illangasekare

대향류 확산화염에서 에지화염이 화염소화에 미치는 영향

대향류확산화염의 화염소화에 있어서 에지화염 역할에 관한 실험적 연구가 진행되었다. 속도비, 버너직경, 그리고 버너간격을 변화시키며 수행된 실험에서 전체신장률에 따른 화염소화 임계질소몰분율의 그래프는 c-커브 형태로 나타났다. 고신장률화염에서는 화염소화 임계질소몰분율의 그래프가 하나의 곡선으로 일치하였으며, 화염이 일차원의 응답특성을 갖는 것을 확인하였다. 화염 소화는 바깥 에지화염이 반경방향으로의 진동 후에 화염 중심으로 수축하며 소화하는 영역, 진동 없이 화염중심으로 수축하며 소화하는 영역, 그리고 바깥 에지부분의 수축과 진동 없이 화염중심에 화염 구멍이 생기며 소화하는 영역으로 세 가지 모드로 나타났다. 화염 표면온도 측정과 에너지 방정식의 각항을 수치해석 한 결과를 토대로 에지화염부분에서의 반경방향 전도 열손실이 에지화염의 불안정을 야기한다는 것과 전도를 통한 열 공급뿐만 아니라 대류를 통한 열 공급도 바깥 에지화염의 안정화에 기여한다는 것을 보였다. 그리고 반경방향의 전도열손실이 수축하며 소화하는 메커니즘의 지배적인 역할을 함을 보였다.

Fusion Yield Enhancement in Magnetized Laser-Driven Implosions

Enhancement of the ion temperature and fusion yield has been observed in magnetized laser-driven inertial confinement fusion implosions on the OMEGA Laser Facility. A spherical CH target with a 10 atm D2 gas fill was imploded in a polar-drive configuration. A magnetic field of 80 kG was embedded in the target and was subsequently trapped and compressed by the imploding conductive plasma. As a result of the hot-spot magnetization, the electron radial heat losses were suppressed and the observed ion temperature and neutron yield were enhanced by 15% and 30%, respectively.

Behavior of Low Strain Rate Flame Disks in Counterflow Diffusion Flame

A study was conducted to clarify flame characteristics through the evaluation of critical mole fractions at flame extinction and edge-flame oscillation of low strain rate flames using the global strain rate, velocity ratio, and burner distance as experimental variables. The transition from a shrinking flame disk to a flame hole was verified through gradient measurements of the maximum flame temperature. Evidence of edge-flame oscillation in flame disks was also found using numerical simulations in zero and normal gravity. The main mechanisms of flame extinction and edge-flame oscillation were analyzed by comparing the energy fractions in the energy equation. For low strain rate flame disks, radial conduction heat loss rather than flame radiation was a significant contributor to flame extinction and even edge-flame oscillation. This was experimentally demonstrated by evaluating the critical mole fraction at flame extinction and edge-flame oscillation, as well as measurements of the flame temperature gradient along the flame disk surface. These results suggest that low strain rate flame responses are determined not only by one-dimensional flame responses, but also by multi-dimensional flame responses such as radial conduction heat loss. The results also show that extinction of low strain rate flames is more probably due to multi-dimensional heat losses than to radiative extinction.

A Study on Flame Extinction Characteristics along a C-Curve

A study has been conducted both experimentally and numerically to clarify impacts of curtain flow and velocity ratio on low strain rate flame extinction and to further display transition of shrinking flame disk to flame-hole. Critical mole fractions at flame extinction are provided in terms of velocity ratio, nitrogen curtain flow rate, and global strain rate. Flame extinction modes are grouped into four on a C-curve, which is characterized by a flame-hole, the shrinking of edge flame, and edge flame oscillation. It is seen that varying curtain flow rate does not impact on edge flame oscillation, flame extinction, and even flame extinction modes. Variation of velocity ratio extends to the low strain rate flame extinction modes beyond the turning point. It is found that the expanding and shrinking flames always have negative flame propagation speed; it is also recognized that the decrease of flame radius is prone to extinguish due to the dominant role of radial conduction heat loss. The examination of ener...