•A downward air jet is used to suppress natural convection from a near-vertical wall.•The jet lowers convective heat loss by ∼30% at the top of the wall, near the jet.•There is a ∼20% reduction in ...heat loss in the laminar-turbulent transition zone.•The concept could be applied to reduce heat loss from solar-thermal receivers.
Concentrating solar power (CSP) plants encounter inefficiencies at all stages of electricity generation. Convection from the solar-thermal receiver is a significant mode of heat loss in CSP systems, and is challenging to mitigate. This study investigates the reduction of convection losses by using a planar jet that disrupts the buoyant flow arising from the heated surface of an external CSP receiver. An isothermal flat plate with a height of 1.8 m was used to model the receiver, and a planar jet air curtain with a nozzle thickness of 3 mm was introduced near the upper edge of the wall. A computational fluid dynamics model was first validated and subsequently implemented to conduct a parametric study on the heat transfer from the isothermal plate with an air curtain varying four parameters: jet speed, jet angle, plate temperature and plate inclination. The results showed that the air curtain generates a stagnation zone adjacent to the wall which successfully reduces local convective heat losses. The effectiveness of an air curtain is defined here as the relative reduction in the local heat loss due to the air curtain, compared to the case of natural convection alone. A local maximum of 31.2% effectiveness is achieved in the stagnation zone below the jet outlet for a vertical wall with a jet speed of 2.5 ms−1 and jet angle of 45∘. The air curtain effectiveness at the stagnation region was found to decrease with increasing jet speeds, whereas the effectiveness increased near the laminar-to-turbulent transition region with increasing jet speed. Smaller air curtain angles relative to the wall resulted in lower effectiveness. A 45∘ air curtain on a vertical wall can offer performance benefits that are similar in magnitude to inclining a wall from the vertical. A higher wall temperature was accompanied by better effectiveness near the jet outlet, particularly in the stagnation region, while lower wall temperatures produced higher effectiveness further from the jet. Therefore, an air curtain can be used to reduce convective heat losses locally from a heated flat surface, including potentially when applied to CSP receivers.
Design of efficient, downsized piston engines requires a thorough understanding of transient near-wall heat losses. Measurements of the spatially and temporally evolving thermal boundary layer are ...required to facilitate this knowledge. This work takes advantage of hybrid fs/ps rotational coherent anti-Stokes Raman spectroscopy (HRCARS) to measure single-shot, wall-normal gas temperatures, which provide exclusive access to the thermal boundary layer. Phosphor thermometry is used to measure wall temperature. Measurements are performed in a fixed-volume chamber that operates with a transient pressure rise/decay to simulate engine-relevant compression/expansion events. This simplified environment is conducive for fundamental boundary layer and heat transfer studies associated with engine-relevant processes. The thermal boundary layer development and corresponding heat losses are evaluated within two engine-relevant regimes: (1) an unburned-gas regime comprised of gaseous compression and (2) a burned-gas regime, which includes high-temperature compression and expansion processes. The time-history of important boundary layer quantities such as gas / wall temperatures, boundary layer thickness, wall heat flux, and relative energy lost at the wall are evaluated through these regimes. During the mild unburned-gas compression, Tcore increases by 30 K and a thermal boundary layer is initiated with thickness δT ~ 200 μm. Wall heat fluxes remain below 6 kW/m2, but corresponds to ~6% energy loss per ms. In the burned-gas regime, Tcore resembles adiabatic flame temperatures, while Twall increases by 16 K. A thermal boundary layer rapidly develops as δT increases from 290 to 730 μm. Energy losses in excess of 25% occur after flame impingement and slowly decay to ~10% at the end of expansion. Measurements also resolve thermal mixing of fresh- and burned gases during expansion, which yield strong temperature reversals in the boundary layer. Findings are compared to canonical environments and demonstrate the transient thermal boundary nature during engine-relevant processes.
Introduction. During construction of underground facilities, architectural, finishing and installation works begin before the completion of the main structures. At that time, there is no permanent ...heating network connection and therefore temporary heat sources must be used to ensure compliance with the works technology. The lack of specific information about the required capacity of such sources makes it impossible to forecast the financial costs, which is essential for contractors. The purpose of the study is to determine the specific value of heat energy consumption for heating underground stations, construction of which is carried out by open method, depending on different outdoor temperatures, construction completion, depth of station installation, as well as the construction volume of the station. Materials and methods. Heat flows have been calculated by means of mathematical modelling of a stationary thermal regime. Analyses of calculation results and normative data have been performed in accordance with the provisions of probability theory and mathematical statistics. Calculation of coefficients and loads are carried out according to generally accepted equations and laws of heat and mass exchange. Climatic data and material parameters are taken according to current normative documents. Results. The values of specific thermal performance of open-cut metro stations depending on space-planning and design solutions, composition of external envelopes, as well as the stage of construction completion (including the presence or absence of backfill) are calculated. Estimated rate of heat energy consumption for temporary heating of open cut metro station is 3.87 Gcal/(1,000 m 3 · month). Conclusions. Temporary heating norms (amount of heat energy) for open-circuit metro stations have been proposed. Heat costs for ensuring the specified air parameters, referred to a unit volume and required for construction and installation works inside the stations during the heating period, are determined.
Animals living in the extremely cold environment, such as polar bears, have shown amazing capability to keep warm, benefiting from their hollow hairs. Mimicking such a strategy in synthetic fibers ...would stimulate smart textiles for efficient personal thermal management, which plays an important role in preventing heat loss and improving efficiency in house warming energy consumption. Here, a “freeze‐spinning” technique is used to realize continuous and large‐scale fabrication of fibers with aligned porous structure, mimicking polar bear hairs, which is difficult to achieve by other methods. A textile woven with such biomimetic fibers shows an excellent thermal insulation property as well as good breathability and wearability. In addition to passively insulating heat loss, the textile can also function as a wearable heater, when doped with electroheating materials such as carbon nanotubes, to induce fast thermal response and uniform electroheating while maintaining its soft and porous nature for comfortable wearing.
A textile with excellent thermal insulation capability, mimicking the porous structure of polar bear hair, is fabricated by a “freeze‐spinning” technique to continuously spin silk fibroin solution into aligned porous fibers. Doped with electroheating materials, this type of textile is beneficial for personal thermal management, thermal stealth in military applications, and wearable electronics.
Heat energy loss in buildings occurs by two means, namely Fabric Heat Loss and Ventilation Heat Loss. Loss occurs when energy flows out through building envelope from inner warmer air to cooler air ...located external to the building. The purpose of this study is to identify and estimate the proportion of Heat Energy Loss that is directly caused by construction components from the building envelope. The novelty of this research is to propose a methodological framework that characterizes the Heat Energy Loss in buildings during the operation phase, taking into consideration the local climate data in which buildings are located. Reliance is on the use of a systematic approach that makes the work readily available to practitioners and experts in the area of energy efficiency. A case example of a single-family house is examined in three different climate classifications for validating the proposed method of this work. Results reveal that Fabric Heat Loss is the main factor of the Heat Energy Loss in buildings; responsible for more than 81% of the total Heat Energy Loss in buildings. Openings and exterior walls play a significant role in curbing such energy loss; accounting for around 70% of the total Fabric Heat Loss in buildings. This work points out that the percentage of energy efficiency improvement of Fabric Heat Loss is similar and directly proportional to the percentage of reduction in U-values of building components; as U-value reduces by 6.66%, the energy efficiency of Fabric Heat Loss improves by 6.66%. Besides, the analysis conducted indicates that lower air change rate would lessen the Ventilation Heat Loss in buildings. Finally, this work illustrates that Heat Energy Loss in tropical climates and dry climates could reach a value of 16% and 8%, respectively, compared to Heat Energy Loss in moist subtropical mid-latitude climates.
•A framework approach to estimate heat energy loss in buildings is proposed.•Heat energy loss is analyzed due to fabric and ventilation heat loss in buildings.•Fabric heat loss of construction components is the main agent of heat energy loss.•Openings and exterior walls play a significant role in curbing fabric heat loss.•Reducing U-value of building components increases energy efficiency in buildings.
Whether whole body heat loss and thermoregulatory function (local sweat rate and skin blood flow) are different between summer and autumn and between autumn and winter seasons during exercise with ...different air flow in humid heat remain unknown. We therefore tested the hypotheses that whole body sweat rate (WBSR), evaporated sweat rate, and thermoregulatory function during cycling exercise in autumn would be higher than in winter but would be lower than in summer under hot-humid environment (32 C, 75% RH). We also tested the hypothesis that the increase of air velocity would enhance evaporated sweat rate and sweating efficiency across winter, summer, and autumn seasons. Eight males cycled for 1 h at 40% V̇o
in winter, summer, and autumn seasons. Using an electric fan, air velocity increased from 0.2 m/s to 1.1 m/s during the final 20 min of cycling. The autumn season resulted in a lower WBSR, unevaporated sweat rate, and a higher sweating efficiency compared with summer (all
≤ 0.05) but WBSR and unevaporated sweat rate in autumn were higher than in winter and thus sweating efficiency was lower when compared with winter only at the air velocity of 0.2 m/s (All
≤ 0.05). Furthermore, evaporated sweat rate and core temperature (
) were not different among winter, summer, and autumn seasons (All
> 0.19). In conclusion, changes in WBSR across different seasons do not alter
during exercise in a hot humid environment. Furthermore, increasing air velocity enhances evaporated sweat rate and sweating efficiency across all seasons.
Numerical study was performed on diffusion H2/air in Y-shaped mesoscale combustors for. After the ignition of fuel/air mixture at the combustor exit, the flame propagates towards the intersection, ...first in premixed flame mode in downstream and shifts to edge flame mode in the upstream. During the transition of flame propagation modes, the change of flame inclination direction was observed.
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•Flame propagates first in premixed flame mode and later shifts to edge flame mode.•The propagation speed of edge flame mode is faster than that of premixed flame mode.•The direction alters in upstream if the flame inclines to the air side in downstream.•The edge flame length decreases as the channel width is reduced.•In fuel lean cases the edge flame length is much shorter in the longer combustor.
Recently, we experimentally investigated the combustion characteristics of non-premixed H2 and air in Y-shaped mesoscale combustors. In the present study, we aim at numerically explaining some experimental phenomena and tendencies, and revealing more details of flame propagation which cannot be well captured in the experiments. The numerical results show that the flame propagates in premixed flame mode in the downstream and shifts to edge flame mode in the upstream, which is a result of the fuel/air mixing effectiveness associated with the longitudinal distance. The inclination direction of flame might change during the propagation process, depending on its original direction after ignition. Moreover, the flame propagates faster in edge flame mode than in premixed flame mode. In addition, the edge flame length increases with the increase of combustor width because a longer distance is needed for a well mixing in the wider channel. Furthermore, the edge flame in the combustor of L = 100 mm is evidently longer compared to that of L = 200 mm when the nominal equivalence ratio is below unity (e.g., ϕ = 0.7). This is because the heat loss ratio under L = 100 mm is less than that of L = 200 mm.
Solar steam generation is regarded as one of the most sustainable techniques for desalination and wastewater treatment. However, there has been a lack of scalable material systems with high ...efficiency under 1 Sun. A solar steam generation device is designed utilizing crossplane water transport in wood via nanoscale channels and the preferred thermal transport direction is decoupled to reduce the conductive heat loss. A high steam generation efficiency of 80% under 1 Sun and 89% under 10 Suns is achieved. Surprisingly, the crossplanes perpendicular to the mesoporous wood can provide rapid water transport via the pits and spirals. The cellulose nanofibers are circularly oriented around the pits and highly aligned along spirals to draw water across lumens. Meanwhile, the anisotropic thermal conduction of mesoporous wood is utilized, which can provide better insulation than widely used super‐thermal insulator Styrofoam (≈0.03 W m−1 K−1). The crossplane direction of wood exhibits a thermal conductivity of 0.11 W m−1 K−1. The anisotropic thermal conduction redirects the absorbed heat along the in‐plane direction while impeding the conductive heat loss to the water. The solar steam generation device is promising for cost‐effective and large‐scale application under ambient solar irradiance.
A low‐cost steam generation device is demonstrated with 80% conversion efficiency under 1 Sun. The heat transfer and fluidic transport directions are decouple, utilizing the mesostructure of wood. The anisotropic thermal conduction redirects the absorbed heat in‐plane while impeding heat loss into bulk water. Nanoscale pits and spirals in crossplanes function as efficient water supply channels.
Increasing the production of energy in line with industry development, transportation, and life quality improvement is an interesting topic needs to be addressed. Energy policymakers and researchers ...have aimed at energy management, particularly by improving energy systems performance. This review paper explains the rising interest of thermoelectric technology and applications. Nowadays, thermoelectric technology such as thermoelectric generators (TEGs) and thermoelectric cooling systems (TECs) provide heat loss recovery of thermodynamic units for power production of remote areas. Unlimited solar energy can also be employed for thermoelectric power production. This paper describes the principles of thermoelectricity and presents an explanation of current and upcoming materials. Developed models and various performed optimization of thermoelectric applications by using non-equilibrium thermodynamics and finite time thermodynamics are discussed as well. Additionally, a number of topical applications and energy resources are introduced. The main goal of this study is to give a clear overview of thermoelectric technology and applications.
•Explains the rising interest of thermoelectric technology and applications.•Describes thermoelectricity principles and current and upcoming materials.•Developed models and various performed optimization are discussed.•Includes non-equilibrium thermodynamics and finite time thermodynamics.•A number of topical applications and energy resources are introduced.
The primary goal of this work is to examine the current materials for insulation utilized in the furnace of a refinery unit and to limit the waste heat dissipation through the radiant wall subbing ...with legitimate insulation materials. There are some exceptional materials whose properties are appropriate for reducing the heat loss in the furnace walls which could clearly decrease the heat loss through the radiant walls. By critical examination of these materials, the external temperature of the furnace is determined to prove that these materials can be utilized as substitution to those of the standard insulation materials.
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