ANALYSIS OF CONVECTIVE HEAT TRANSFER DURING IMMERSION FRYING

Immersion frying was studied as a high temperature drying process and analogies drawn between periods found in drying and similar periods in immersion frying. Analysis of external heat transfer during immersion frying showed a highly complex system of free and forced convection augmented by boiling conditions. Oil flow was found to be driven downward by buoyancy forces due to cooling at the sample surface then upward due to entrainment in vapor bubbles during boiling conditions. Experimental work showed the convective heat transfer coefficient to be a dynamic property ranging from 300 to 1100 W/m²°C and to be strongly coupled with bulk movement of the oil. Heat flux measurements found a peak flux of nearly 30,000 W/m³. Based on analysis of bubble dynamics it is hypothesized that heat flux increases with increased oil degradation through a reduction in vapor bubble size and increase in bubble frequency due to changes in interfacial properties of the oil.     

Dimensionless Formulation of Convective Heat Transfer in Fry‐Drying of Sewage Sludge

Fry‐drying is an alternative for heat and mass transfer intensification. The process reuses waste oil as a heating medium for drying by contact with the wet sludge. At the end of the process, a stable derived fuel is obtained, a granular solid composed of the dried indigenous sewage solid and the impregnated oil. The fry‐dried sludge is storable and transportable without any pathogen elements. Knowledge about heat and mass transfer rates during the frying process is essential in order to assess the quality of the final product such as calorific value, oil uptake, porosity changes, etc. The heat transfer properties including transfer by free convection between the solid and the frying oil are fundamental for the process design and manufacturing of the fry‐dried product. The convective heat coefficient by temperature measurement and overall energy balance calculation is determined. The heat flux is calculated from the fry‐drying kinetics including moisture loss and oil intake kinetics. Various hydrodynamic regimes for convective heat transfer during the frying process are discussed (non‐boiling, boiling, and low‐boiling regime). A dimensionless formulation for estimating the convective transfer is proposed.     

Drying Characteristics In Superheated Steam Drying at Reduced Pressure

Abstract

The superheated steam drying at reduced pressure is performed, and the effects of operational conditions such as drying pressure and temperature on the drying characteristics are examined. In order to obtain the basic guideline for the design of the superheated steam dryer at reduced pressure, the heat flux to sample was calculated and the optimal conditions were estimated.

After the sample temperature reached at the boiling point, the temperature was maintained at the boiling point and the drying rate became almost constant. Once the sample was dried out, the temperature suddenly increased up to the drying gas temperature. From the calculation of combined heat flux, the followings were found. The contribution of radiative heat transfer to the combined heat flux became larger as the drying pressure was lower. The combined heat flux had a maximum value against the drying pressure. The optimum drying pressure, which gave the maximum heat flux, became lower as the drying gas temperature decreased. It was found that reduction in the drying pressure is effective for the enhancement in drying performance.     

烧结型多孔表面管外池沸腾传热特性

实验研究了热通量为0.1~160 kW·m^-2时,去离子水在光管及烧结型多孔表面管管外的池沸腾传热特性,分析了换热管布置方式(垂直与水平)、管径大小(20、25和32 mm)与多孔层颗粒尺寸(30~105 μm)对池沸腾传热特性的影响规律。结果表明:去离子水在多孔管表面的起始沸腾过热度小于光管,比光管低3 K左右;多孔表面管可明显强化核态沸腾传热,其沸腾传热系数可达光管的3~4.5倍;大热通量下,换热管水平布置时的传热效果较垂直布置佳,且布置方式对多孔管换热效果的影响比对光管的影响小;随管径增大,光管与多孔表面管的沸腾传热系数降低;大颗粒尺寸多孔层的强化效果优于小颗粒尺寸多孔层。     

Enhanced Boiling of Methanol on Copper Coated Surface

This paper presents an experimental investigation on nucleate pool boiling of saturated methanol on uncoated as well as copper‐coated mild steel heating tubes of various thicknesses at atmospheric and subatmospheric pressures. It also includes the effect of heat flux, pressure and coating thickness on the boiling heat transfer coefficient of coated tube surfaces. Heat flux was progressively increased from 15,670.20 to 43,151.57 W/m² in six steps and pressure from 23.02 to 98.68 kN/m² in five steps. Boiling characteristics on such surfaces are compared with those on uncoated ones. Also, a criterion for enhanced boiling of methanol on copper‐coated tubes is described.     

Fouling of tube bundles under pool boiling conditions

This paper aims to provide experimental heat transfer results for boiling of CaSO₄ solutions on the outside of tube bundles, and also to shed some light into the mechanisms which influence heat transfer fouling under pool boiling conditions. Heat transfer coefficients for three heater rods with an identical diameter of 10.67 mm on a 35 mm vertical pitch have been measured. The independent variables bulk concentration and heat flux have been varied from 0.8 to 1.6 g/L and 33 to 300 kW/m², respectively. The experimental results show that the mechanisms of fouling on the middle and top heater substantially differ from those at the bottom heater, due to the dominant effects of bubble impingement on the heat transfer surfaces. In particular, the competing effects on the degree of wall superheat and the supersaturation of the boiling liquid play a significant role. Spalling effects are more pronounced on the upper heaters where thin, friable deposits were found with low density and adherence, particularly at higher heat fluxes and lower concentrations. In terms of tube bundle efficiency, the best results were found for lower concentrations.     

Heat and Mass Transfer During Fry-Drying of Sewage Sludge

Deep-frying, which consists of immersing a wet material in a large volume of hot oil, presents a process easily adaptable to dry rather than cook materials. A suitable material for drying is sewage sludge, which may be dried using recycled cooking oils (RCO) as frying oil. One advantage is that this prepares both materials for convenient disposal by incineration.

This study examines fry-drying of municipal sewage sludge using recycled cooking oil. The transport processes occurring during fry-drying were monitored through sample weight, temperature, and image analysis. Due to the thicker and wetter samples than the common fried foods, high residual moisture is observed in the sludge when the boiling front has reached the geometric center of the sample, suggesting that the operation is heat transfer controlled only during the first half of the process followed by the addition of other mechanisms that allow complete drying of the sample. A series of mechanisms comprising four stages (i.e., initial heating accompanied by a surface boiling onset, film vapor regime, transitional nucleate boiling, and bound water removal) is proposed. In order to study the effect of the operating conditions on the fry-drying kinetics, different oil temperatures (from 120 to 180°C), diameter (D = 15 to 25 mm), and initial moisture content of the sample (4.8 and 5.6 kg water·kg^?1 total dry solids) were investigated.     

超亲/疏水性表面池沸腾传热研究

为研究超亲/疏水性表面对沸腾传热的影响,用H₂O₂氧化的方式制备了超亲水表面,用氨水加高分子修饰的方式制备了超疏水表面。在常压下以蒸馏水为工质,采用高速摄影仪对其进行了池沸腾传热实验。结果表明,超疏水表面亲气疏水,在沸腾起始点易于产生气泡,且气泡不易脱离,此时壁面过热度ΔTs仅为2.4K,但随热流密度的增大,气泡易于聚合,所产生的大气泡阻碍了传热的进行,传热开始恶化,临界热流密度（CHF）较低;而H₂O₂氧化的表面由于刀片状微纳结构的存在,增加了表面的粗糙度,不仅增大了相变传热表面积、增加了核化点数量,而且具有超亲水特性,气泡脱离频率较大,大大强化了沸腾传热,最大换热系数约是光表面的1.7倍,且相应地提高了CHF,可达131.0W/cm²,表现出较好的传热特性。     

Heat transfer between FCC catalyst and an electrically heated horizontal cylinder in a circulating fluidized bed

The heat transfer coefficient between a suspension of FCC particles and a horizontal cylindrical heat transfer probe inserted into the riser or the standpipe of a CFB has been quantified. With the heat transfer probe located in the riser 4.75 meters above the L-valve, and solids mass flux varied between 0 to 100 kg/(m²·s), the heat transfer coefficient ranged from 70 W/(m²·K) to 475 W/(m²·K). On a plot of heat transfer coefficient versus solids mass flux, three zones have been identified, which correspond to the difference in the flow structure of the solids around the heat transfer probe as the solids mass flux increases. Also, measurements were taken of the radial solids flux in two orthogonal directions using an isokinetic sampling system. The data shows the asymmetry due to the perturbations introduced by the heat transfer probe. Finally, the heat transfer in the downcomer was investigated. It has been found that the magnitude of the heat transfer coefficient in the downcomer is dominated by the solids flux; variation in gas bypassing in the standpipe has little effect. Results obtained by traversing the heat transfer probe across the diameter of the standpipe suggest that the heat transfer coefficient is nearly independent of radial position within the standpipe.     

氧化石墨烯表面的饱和池沸腾强化传热实验

利用氧化石墨烯（GO）纳米片沸腾自组装法（self-assembly）制备出GO纳米表面,以蒸馏水为液体工质,对常压下GO纳米表面和光滑铜平面的饱和池沸腾换热特性进行了对比实验研究,并用高速摄像机拍摄了汽泡的动态行为。结果表明,GO纳米表面降低了换热壁面的过热度,其临界热流密度（CHF）和换热系数（HTC）分别达到了208W/cm²和7.25W/(cm²?K),较光滑铜平面分别提高了66.4%和86.9%。分析认为,是铜基底表面沉积的润湿性优异的高导热二维GO层状结构促使了CHF提高。汽泡可视化观察发现,相比于光滑铜平面,较低热流密度时,相同热流下GO纳米表面上汽泡脱离直径较小,脱离频率较高,汽化核心增多;较高热流密度时,光滑铜平面汽泡合并现象更严重,即GO纳米表面能延缓导致CHF产生的表面蒸汽膜的出现。     

纳米制冷剂管内流动沸腾换热特性

研究了CuO-R113纳米制冷剂在水平直光管内的流动沸腾换热特性。实验测试段长度1.5 m、外径9.52 mm。实验工况的质量流率为100～200 kg•m^-2•s^-1,热通量为3.08～6.16 kW•m^-2, 入口干度为0.2～0.7,纳米颗粒质量分数为0～0.5%。结果表明：CuO-R113纳米制冷剂的传热系数高于纯R113制冷剂的传热系数。纳米颗粒的加入,强化了制冷剂管内流动沸腾换热。质量流率为100、150、200 kg•m^-2•s^-1的情况下,传热系数分别最大提高了29.7%、22.7%、25.6%。     

朝下沟槽结构表面池沸腾换热

核电站熔融物堆内滞留技术是一项关键的严重事故应对策略,该策略已被核电站广泛采用。为增强核电站压力容器下封头外表面的沸腾换热能力,实验研究了常压下朝下沟槽结构表面的池沸腾换热,测量了倾角5°、30°、45°、60°和90°下热通量随壁面过热度的变化,获得了相应倾角下的临界热通量（CHF）。与光表面相比,朝下沟槽结构表面的CHF可提高65%～90%。实验观察发现,在高热通量下朝下沟槽结构表面气泡运动形态存在蒸汽膜和波浪蒸汽层两种结构。分析表明,沸腾换热显著增强、临界热通量大幅提高的原因是沟槽结构大幅增加了换热面积同时还明显改善了表面的润湿性。     

Ébullition par Convection Forcée des Mélanges Eau‐Ammoniac dans un Tube Vertical

An experimental study has been conducted on the forced convective boiling heat transfer of ammonia‐water mixtures flowing inside a 6 mm inner diameter vertical smooth tube. Using a water‐heated double pipe type generator, the local heat transfer coefficients are measured inside the inner tube for a range of heat flux density (29.93 — 99.79 kW/m²), mass flux density (35.36 — 99.04 kg/m²·s), mass flow rate (0.001 — 0.03 kg/s) and ammonia mass concentration (49%, 55% and 61%). The effect of the experimental parameters on the heat transfer coefficients is analysed. Three methods are used to predict the boiling heat transfer coefficients. Experimental data were compared with the available correlations. The obtained results confirm the good performance of the Mishra et al. (1981) and Bennett‐Chen's (1980) correlations in predicting the convective boiling heat transfer coefficient of NH₃‐H₂O mixtures. These methods are able to predict the boiling heat transfer data within an average accuracy of ± 20 %.     

The Gas-to-Particle Heat Transfer and Hydrodynamics in Spouted Bed Drying of Sawdust

Abstract

This article presents experimental results for spouted bed drying of sawdust, carried out in a full-scale as well as in a laboratory-scale dryer using air as well as steam as drying media. The aim is to present design parameters for a spouted-bed sawdust dryer that can be used by the industry in designing full-scale dryers. A hydrodynamically stable spouted jet spouted bed was obtained. The heat transfer characteristics of the bed were represented in terms of a volumetric heat transfer coefficient (VHC). When sawdust is dried in a spouted bed, the mean VHC is increasing up to fiber saturation level (20–25% wb) from 40 to 110 W/m³ K. The VHC decreases with the residence time and with an increased static bed height. Gas temperature profiles are also presented for the bottom part of the drying chamber.     

Pore Development and Moisture Transfer in Chicken Meat during Deep-Fat Frying

Abstract

Changes in the structure of food products play important role in the various mass transfer processes during deep-fat frying. The relationship between moisture loss and pore formation were investigated at frying oil temperatures of 170, 180, and 190°C and frying times up to 900 s. Porosity and pore structure were characterized by using mercury intrusion porosimetry and helium displacement pycnometer. Moisture transfer in the samples was modeled using Fick's law and effective moisture diffusivity was computed from experimental data. Pore formation changes significantly (P < 0.01) in time as modulated by frying oil temperature. A peak pore fraction of 0.283 (after 360 s of frying), 0.238 and 0.220 (after 900 s of frying) at frying temperatures 190, 180 and 170°C, respectively was observed. Effective moisture diffusivity of 5.4 to 6.9 × 10^?9 m² s^?1 and activation energy of 20 kJ/mol was obtained for the frying oil temperatures. Changes in pore structure influenced moisture diffusivity and oil uptake. Eighty-four percent of the pores are capillary pores, hence moisture transfer increased.     

多级复合芯结构的强化沸腾传热研究

采用固相烧结技术制备了均匀多孔层、复合16芯和复合32芯三种多孔结构,并且建立了池沸腾传热测试系统来研究不同芯数量、粒径与结构高度对多孔结构沸腾传热性能的影响。实验结果表明,在测试范围内复合层高1 mm的多孔复合32芯结构传热性能较强,临界热通量(CHF)最高为386 W/cm²,传热系数最高达到9.5 W/(cm²·K)。同时利用高速摄影观察气泡行为来研究强化沸腾传热机理。可视化数据表明,相比于光滑表面,在高热通量下多孔复合表面上气泡周期更短,脱离更快,气泡的离开带来了更多的液体补充,进而不断提升传热性能,获得更高的CHF值。     

电场强化乙醚自然对流和池沸腾换热

外加电场强化传热是将电场及其理论引入传热学领域,利用电场力与流场和温度场的相互作用而达到强化传热的一种有效方法．     

Enhanced Heat Transfer Characteristics of Upward Flow Boiling of Kerosene in a Vertical Spirally Internally Ribbed Tube

Experiments of upward flow boiling of kerosene in a vertical spirally internally ribbed tube and a vertical smooth tube were conducted, respectively, in the present study. The spirally internally ribbed tube has an inner diameter of 11 mm (an equivalent inner diameter of 11.6 mm) and an outer diameter of 22 mm. The smooth tube has an inner diameter of 15 mm and an outer diameter of 19 mm. The test tubes were uniformly heated by passing an electrical current along the tubes with an available heated length of 2500 mm. At the outlet of the test section, the experimental pressure was 3 bars. The experimental heat flux ranged from 28.5 to 93.75 kW/m². The experimental mass flux was 410, 610, and 810 kg/m²s, respectively. Both local and average flow boiling heat transfer coefficients were measured in the test tubes. The enhanced heat transfer characteristics of the flow boiling of kerosene in the spirally internally ribbed tube are presented by comparing the experimental heat transfer coefficients with those obtained in the smooth tube. It shows that the flow boiling heat transfer coefficients in the spirally internally ribbed tube are 1.6 to 2 times greater than those in the smooth tube. The physical mechanisms of the enhanced heat transfer characteristics of flow boiling in the spirally internally ribbed tube are analyzed. According to the experimental data, an expression for the flow boiling heat transfer coefficient of kerosene was found in terms of the Martinelli number for the spirally internally ribbed tube. The correlation is applicable to the design of heat exchange equipment, using the spirally internally ribbed tube as a heat transfer element under these test conditions.     

Analysis the fry-drying process of oily sludge sample

Oily sludge is one of the hazardous materials if not properly treated. Thus, recovering oil from oily sludge could reduce environmental problems and have substantial commercial benefits. However, prior to oil recovery, dehydration is extremely necessary to effectively reduce energy consumption. Fry-drying is a novel dehydration method which characterized by low energy consumption and high drying efficiency. In this study, the spent lubricating oil of vehicle was used as frying medium, which enabled a drying operation that was environmental friendly and economically competitive. A modified Dean-Stark apparatus-II was designed to accurately and efficiently measure the water and oil contents of the fried sample. Considering the oil adsorption mechanism that occurred during the fry-drying of oily sludge, a suitable equation for the forced convective heat transfer coefficient (h) was established using the fundamental of energy balance. Results showed that the h change tendency and the calculated maximum value were distinct from previous food frying findings. Finally, the entire fry-drying process of oily sludge was divided into four periods according to the different sample drying rate change tendencies. The heat and mass transfer processes of each period were also analyzed.     

R-113池沸腾气泡行为的可视化及传热机理

利用ITO透明加热膜和高速摄影仪,对制冷工质R-113在压力为0.1 MPa的池沸腾的气泡行为进行了可视化研究.利用高速摄影仪观察得到了不同热通量下气泡的生长、脱离以及相邻气泡之间的合并情况.实验结果表明,随着热通量的升高,气泡的脱离时间减小,而加热壁面的汽化核心密度增大.文中给出了气泡的脱离时间、脱离直径以及汽化核心密度随热通量的变化曲线.在实验过程中,并没有观察到气泡底部的微液层在蒸干过程中有液体补充.最后,利用动态微液层模型对R-113的池沸腾换热曲线进行了预测,并将预测结果和实验结果进行了对比,结果显示,预测结果与实验结果符合较好.