Magnitude Of Heat Loss Research Articles

A thick reaction zone model for premixed flames in two-dimensional channels

Direct interactions between the flow field and the chemical reaction in premixed flames occur when the reaction zone thickness is comparable to, or greater than flow length scales. To study such interactions, a laminar model is considered that has direct bearings to steadily propagating deflagrations in a Hele-Shaw channel with a background plane Poiseuille flow. The study employs asymptotic analyses, pertaining to large activation energy and lubrication theories and considers a distinguished limit where the channel width is comparable to the reaction zone thickness, with account being taken of thermal-expansion and heat-loss effects. The reaction zone structure and burning rates depend on three parameters, namely, the Peclet number, , the Lewis number, and the ratio of channel half-width to reaction zone thickness, . In particular, when the parameter is small wherein the reaction zone is thick, transport processes are found to be controlled by Taylor's dispersion mechanism and an explicit formula for the effective burning speed is obtained. The formula indicates that for , which interestingly coincides with a recent experimental prediction of the turbulent flame speed in a highly turbulent jet flame. The results suggest that the role played by differential diffusion effects is significant both in the laminar and turbulent cases. The reason for the peculiar dependence can be attributed, at least in our laminar model, to Taylor dispersion. Presumably, this dependence may be attributed to a similar but more general mechanism in the turbulent distributed reaction zone regime, rather than to diffusive-thermal curvature effects. The latter effects play however an important role in determining the effective propagation speed for thinner reaction zones, in particular, when is large in our model. It is found that the magnitude of heat losses at extinction, which directly affects the mixture flammability limits, is multiplied by a factor in comparison with those corresponding to the no-flow case in narrow channels.

Enhancing the Efficiency of Integrated Energy Systems by the Redistribution of Heat Based on Monitoring Data

Integrated energy systems (IES) for combined power, heat and refrigeration supply achieved a wide application due to high flexibility in matching current loading. So as electricity is easily convertible into any other form of energy, gas engines are widely applied as driving engines characterized by high electrical and overall efficiency of about 45% and 90%, respectively. However, the highest thermal efficiency is achieved at full matching heat generated by the engine and heat transformed. This is often impossible in actual practice, especially if the heat is transformed into refrigeration by the most efficient and widespread absorption lithium-bromide chillers (ACh) and the heat not consumed by the ACh is removed from the atmosphere through an emergency radiator. The unused heat might be transformed by an ejector chiller (ECh) as the simplest and cheapest. So as the thermodynamic efficiency of any combustion engine is influenced essentially by the sucked air temperature, the excessive refrigeration produced by the ECh, is used for IES cooling to generate additional electricity and increase the electrical and overall efficiency of the engine. Such a redistribution of heat enables the enhancement of the efficiency of IES with an absorption-ejector chiller (AECh). The modified criteria for the comparative estimation of thermodynamic efficiency of innovative IESs with AEChs without overgenerated heat lost against a typical IES with an ACh and heat lost are proposed. In contrast to well-known electrical and heat efficiency, it considers the magnitude of heat loss and enables us to compare the heat efficiency of any version of transforming heat to refrigeration with an ideal basic version of IES based on a highly efficient ACh, transforming all the heat removed from the engine without heat loss. Some alternative scheme decisions for heat recovery systems have been developed based on monitoring data. They might be easily implemented into a typical IES with ACh.

Pipe Sizing for Novel Heat Distribution Technology

This paper assesses pipe sizing aspects for previously proposed, novel, low heat distribution technology with three pipes. Assessment issues include heat loss, pressure loss, and pipe sizing for different typical pipe configurations. This assessment has been provided by the analysis of a case area with single-family houses. Concerning heat loss, the proposed three-pipe solutions have the same magnitude of heat loss as conventional twin pipes, since lower return temperatures compensate for the larger heat loss area from the third pipe. Regarding pressure loss, the main restriction on the size of the third pipe is limited to the pressure loss in the third pipe. Thermostatic valves to manage the flow rate of the third pipe are advocated, since alternative small pumps have not been found to be commercially available. The pipe sizing recommendation is that the third pipe for recirculation purposes can be two to three standard pipe sizes smaller than the corresponding supply and return pipe, if no prosumer is connected in the heat distribution network.

Electrothermal Simulation-Based Comparison of 4H-SiC p-i-n, Schottky, and JBS Diodes Under High Current Density Pulsed Operation

Pulsed power applications are characterized by very high instantaneous power due to the high voltage and current involved. Power diodes used as a closing switch in pulsed power circuits must be able to withstand the high current operation well above the continuous device rating, for a transient duration. Due to the superior electrothermal properties of wide bandgap Silicon Carbide (SiC) material, it is feasible to develop high Blocking Voltage (BV) Schottky and Junction Barrier Schottky (JBS) diodes besides p-i-n rectifiers. In order to evaluate the device performance under high current density pulsed operation, 2-D models of SiC p-i-n, Schottky, and JBS diodes rated for 3.3-kV BV and 100 A/cm2 current density were developed using Silvaco ATLAS TCAD software. The diode structures were simulated electrothermally to study the device behavior and compare the performance under high current density pulsed operation. The power dissipation and the lattice temperature profile of the SiC diodes were analyzed to compare the magnitude of heat loss and formation of thermal hot spot in the diode structure to predict the suitability of the device for pulsed power applications.

Effect of heat loss on laminar flamelet species concentration

The effects of heat loss on the structure of laminar flamelets, which are the constitutive elements of turbulent flames under the most common operating conditions, are investigated for typical aeronautical gas-turbine operating conditions at take-off. The magnitude of heat loss is quantified via the “enthalpy defect” measured with respect to an adiabatic flame. A procedure to generate laminar flamelets with an assigned enthalpy defect at the boundaries is devised and applied to nonpremixed propane/air flames, as propane reproduces the essential features of higher hydrocarbon combustion. It is found, contrary to commonly held beliefs, that the enthalpy defect has a significant effect on the concentration not only of minor species, but also of main reaction products. Such effects are found in general to be more pronounced for fuel-rich conditions. An impact is anticipated on the formation rate of nitric oxides. The effects of scalar dissipation rate are also discussed.

나노유체를 이용한 평판형 태양열 집열기의 효율에 관한 연구

본 논문에서는 나노유체를 사용한 평판형 집열기의 효율을 예측하기 위한 이론적인 연구를 수행하였다. 평판형 태양열 집열기 내부의 온도분포에 관한 해석적 해를 구하기 위해 무차원화된 2 차원 열확산방정식을 풀었으며, 이 과정에서 흡광계수와 복사강도는 파장에 독립적이라고 가정하였다. 이렇게 주어진 식을 바탕으로, 물-기반 단일벽 탄소나노혼 나노유체를 작동유체로 사용할 경우 나노입자의 부피비, 열손실의 크기, 집열기의 높이에 따른 무차원 온도분포를 파악해 보았다. 마지막으로 나노유체 기반 평판형 집열기의 효율을 예측해 본 결과 일정 형상조건 이내에서 나노유체 태양열 집열기가 기존 집열기 보다 높은 효율을 가질 수 있음을 파악하였다.

Gasdynamic stimulation of combustion of lean fuel mixtures. 2. Evaluation of thermodynamic parameters and the magnitude of heat losses

Various gasdynamic regimes of combustion of a lean fuel mixture of propane with air, including combustion of the mixture at rest, combustion with annular twisting of the gas, and combustion stimulated by jets of a burning gas injected into the combustion chamber (PJC) were analyzed from the viewpoint of the magnitude of heat losses. The advantage of PJC over other combustion regimes is shown. We suggest a simple method to evaluate the pressure increase by calculating the magnitude of the energy release with allowance for heat losses to the combustion-chamber walls.

Effect of phase velocity nonequilibrium on flame propagation and extinction

The problem of flame propagation in the presence of inert particles is considered. The mathematical model used is based on the equations of the mechanics of multiphase media in a two-temperature, two-velocity approximation. Numerical calculations of combustion in the presence of a dispersed phase in zero gravity show that the flame does not quench. For flame propagation in the gravity field directed toward falling particles, a new phenomenon, namely, flame quenching, is revealed. It is shown that the quenching is due to the occurrence of feedback between the flame velocity and the magnitude of heat losses. Critical parameters for various values of the external mass force and particle size are determined.

Heat losses associated with continuous venovenous haemodiafiltration in intensive care patients

Objective: To determine the magnitude of heat loss associated with the use of continuous venovenous haemodiafiltration (CVVHD), and the effect of warming the dialysate and insulating the circuit. Design: Extracorporeal temperature decrease was measured by thermocouples inserted into injectate sensor housings across the standard CVVHD circuit, and across circuits with dialysate warmed in a 40°C bath and/or insulated by aluminium foil. Setting: Intensive Care Unit (ICU) of a tertiary referral centre. Subjects: Twelve critically ill patients undergoing CVVHD. Main results: Mean extracorporeal temperature decrease using a standard circuit was 4.4°C (95% confidence interval [CI], 3.9-4.7), 3,000 kJ (710 kcal) per day would be required on average to maintain thermal homeostasis. Warming dialysate fluid reduced mean temperature decrease to 2.9°C (95% CI, 2.7-3.2), but aluminium foil insulation did not have a significant effect. Conclusion: Heat loss is a significant proportion of expected daily energy requiremen...

Flow microcalorimeter for heat capacities of solutions

A new type of flow microcalorimeter for measuring heat capacities at constant pressure of liquids and solutions was constructed. This calorimeter is the similar in design to Picker's except for the flow system, which consists of two syringe type of pumps and two flowing paths in each flow cell. It was found that the magnitude of heat loss from cells depended on liquids themselves used and the flow rates of sample liquids. The molar heat capacities, C p of benzene and ethanol were determined relative to those of cyclohexane and water, respectively. The excess molar heat capacities, C p (E) for the systems of benzene + cyclohexane and water + ethanol were also determined at 298.15K by the direct mixing method. An inaccuracy for C p (E) was estimated to be within ± 1%.

The Influence of Hardarian Gland Removal and Fur Lipid Removal on Heat Loss and Water Flux to and from the Skin of Muskrats, Ondotra zibethicus

The Hardarian gland (an apocrine organ located behind the eye) was removed from one group of muskrats. Another group of muskrats was anesthetized and shampooed. The fur of shampooed and Hardarianectomized muskrats did not appear to repel water as effectively as sham control animals. Both shampooed and Hardarianectomized muskrats had higher rates of heat loss while submerged in a 4 C water bath than sham control animals. The magnitude of heat loss was less for the Hardarianectomized muskrats. Hardarianectomized and shampooed muskrats had elevated rates of evaporative water loss with an estimated cutaneous water loss of 52% and 66% of total evaporative water loss for Hardarianectomized and shampooed muskrats compared with 41 % for control animals. Coating the fur of muskrats with mineral oil tended to retard evaporative water loss. It appears that the Hardarian gland is an effective source of lipids which may be spread onto the fur from the nasal duct by both self and communal grooming. This lipid coat appe...

Heat Losses from a Divided-Chamber Diesel Engine

The various components of heat loss were investigated in a single-cylinder divided-chamber diesel engine. The effects of engine speed, air/fuel ratio, injection timing, intake air temperature and coolant temperature on the heat rejection to the coolant were examined. The magnitudes of heat rejections to the coolant and to the individual cooling zones (cylinder liner, intake port, exhaust port and antechamber) were found to be related primarily to the rate of fuel consumption. The contributions of piston friction and exhaust port heat transfer to heat rejection to the coolant and the magnitude of heat loss from the exterior surfaces of the engine to the surroundings were also evaluated.

Respiratory heat transfer in cold water and during rewarming.

Respiratory heat loss was measured during cold water (5-6 degrees C) excursions to depths of 300 m. Losses were computed with and without respiratory gas heating when wearing a diving helmet in current commercial and Service use. A magnitude of heat loss, sufficient to cause undue stress to the respiratory tract, was observed even with gas heating provided, particularly at maximum depth. If the hyperbaric gas is both heated and humidified to a maximum comfort level then respiratory gains in the order of 40 W are possible at 250 m. This technique of utilizing warm wet gas introduced into the respiratory tract was tested as a rewarming technique following immersion in cold water (4-5 degrees C) to the limit of peripheral endurance.

A heat-loss index incorporating solar, temperature wind and cloud cover effects

The heating-degree-day concept applied by industry and energy planners for the purpose of evaluating heating requirements is used extensively. However, the heating-degree day, a measure of the departure of the daily mean temperature from a base temperature, neglects the effects of wind, insolation and cloud cover. It has been established that wind speed has a considerable effect on the magnitude of heat loss on man and on enclosed spaces [1 – 5, and others]. Consequently, the heating-degree-day concept is incomplete in evaluating heating requirements and as Ayres [6] in his study on energy consumption of four buildings has observed, “The lack of correlation (of energy consumption) with degree-days also points out the inherent weakness of calculation procedures based on outside-air dry-bulb variations only.” In addition, solar radiation input and long-wave radiative losses are also important factors which must be considered, as they may represent a large heat source or sink. An attempt is made, in this study, to develop a heat-loss index model which incorporates the combined effects of temperature, wind, insolation and long-wave radiative heat loss.