Potential of integrating Na2SO4 · 10H2O pellets in solar drying system

In the current study, evolution of thermophysical properties of red chilli dried in a mixed mode solar dryer that integrates sodium sulfate decahydrate (Na₂SO₄?·?10H₂O) and sodium chloride (NaCl) as thermal storage were presented. Solar drying with Na₂SO₄?·?10H₂O reduced the drying time by 26.7 and 39%, compared to the drying time with or without NaCl. Dimensional shrinkage was gradual with a nonlinear exponential shape for the whole drying conditions. The evolution of the bulk and particle densities decreased while the porosity of the seed increased with time. The coefficient of heat and mass transfer varied from 0.0036???0.035?W/m²?K to 6.09?×?10^?9???6.2?×?10^?8?m/s, respectively. The thermal conductivity, specific heat capacity, and thermal diffusivity ranged from 0.0568 to 0.1093?W/m?K, 1,072 to 2218.7?J/kg?K, and 4.7?×?10^?5 to 5.13?×?10^?5?m²/s, respectively.     

Thermal conductivities from temperature profiles in the polymer electrolyte fuel cell

We report measured temperatures inside the single polymer fuel cell, and thermal conductivities and heat transfer coefficients calculated from these. Temperatures were measured next to the membrane on its two sides, and in the gas channels. Higher temperatures (5 °C or more at 1 A/cm²) were found at the membrane electrode surface than in the gas channels. The thermal conductivity of the membrane (λ^m) was small, as expected from the properties of water and polymer, while the heat transfer coefficient of the electrode surfaces (λ^s) was smaller, 1000±300 W/m² K for a layer thickness of 10 μm. The real coefficient is smaller, since the measured temperatures are systematically smaller than the real ones. The electrode surface heat transfer coefficient is not previously reported. The average value for the catalyst surface plus gas diffusion layer was 0.2 W/m K.     

Analysis of the Shrinkage Effect on Mass Transfer During Convective Drying of Sawdust/Sludge Mixtures

Convective drying of wastewater sludges and sawdust/sludge mixtures was studied. The first part of this work was an experimental study performed in a cross-flow convective dryer using 500 g of wet material extruded through a disk with circular dies of 12 mm. The results showed that the sawdust addition has a positive impact on the drying process from a mass ratio of 2/8, on a dry basis, with observed drying rates higher than the original sludge. The second part of this work consisted of developing a drying model in order to identify the internal diffusion coefficient and convective mass transfer coefficient from the experimental data. A comparison was made between fitted drying curves, well represented by the Newton's model, and the analytical solutions of the diffusion equation applied to a finite cylinder. Variations of dimensional characteristics, such as the volume and exchange surface of the sample bed, were obtained by X-ray tomography. This technique allowed us to confirm that shrinkage, which is an important phenomenon occurring during sludge and sawdust/sludge mixture drying, must be taken into account. The results showed that both the internal diffusion coefficient and convective mass transfer coefficient were affected by mixing and sawdust addition. The internal diffusion coefficient changed from 7.77 × 10^?9 m²/s for the original sludge to 7.01 × 10^?9 m²/s for the mixed sludge and then increased to 8.35 × 10^?9 m²/s for the mixture of a mass ratio of 4/6. The convective mass transfer coefficient changed from 9.70 × 10^?8 m/s for the original sludge to 8.67 × 10^?8 m/s for the mixed sludge and then increased to 12.09 × 10^?8 m/s for the mixture of a mass ratio of 4/6. These results confirmed that sawdust addition was beneficial to the sludge drying process as the mass transfer efficiency between the air and material increased. Reinforcing the texture of sludge by adding sawdust can increase the drying rate and decrease the drying time, and then the heat energy supply will be reduced significantly. The study also showed that neglecting shrinkage phenomenon resulted in an overestimation for the internal diffusion coefficient for the convective drying of sludges and sawdust/sludge mixtures.     

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.     

Study of the kinetics of sewage sludge pyrolysis using DSC and TGA

Thermogravimetry and differential scanning calorimetry were used to examine, in nitrogen, four sewage sludges at modest heating rates. A mechanism consisting of two independent reactions was derived for an undigested sludge. The parameters are: n₁ = 10, E₁ = 130 kJ mol^?1, A₁ = 1 × 10¹⁸s^?1, n₂ = 15, E₂ = 250 kJ mol^?1, A₂ = 1 × 10²⁵s^?1, with initial weight fractions of W₁ = 0.20, W₂ = 0.43, the remainder being non-reactive ash. Analysis was performed by comparing Friedman multiple-heating-rate analyses of experimental and model curves in an iterative manner. DSC experiments provide an understanding of the pyrolytic reaction mechanism and heat transfer in the thermogravimetric analyser. DSC analysis of sewage sludge was sensitive to decomposition reactions of small quantities of organic salts in the sludge. Sewage sludge could be a profitable pyrolysis feedstock if mixed with municipal solid waste.     

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.     

HYDRODYNAMICS AND MASS TRANSFER IN A SPOUTED BED DRYER

ABSTRACT

The disposal of sludge generated by water treatment plants poses major financial and environmental problems. Drying of the sludge is an essential pan of any disposal process which may include incinerating, landfilling or upgrading. In the present study, experiments were carried out to investigate the drying of sludge in a spouted bed and to characterize the hydrodynamics and mass transfer mechanisms. The effect of bed moisture content on the minimum spouting velocity (U_ms) was examined for sludge granules. U_ms was found to increase with increasing the moisture content of the sludge particles. The gas phase mass transfer coefficient in the dryer was determined using porous calcined alumina particles and the results were compared with predictions by correlations available in the literature. The existing correlations gave poor predictions for the mass transfer coefficient. A new correlation for the mass transfer coefficient in a spouted bed dryer was developed based on the experimental results.     

Heat transfer between a gas-solid fluidized bed and a small immersed surface

In this paper are presented the results of an investigation of heat transfer from a small electrically-heated element to the bulk of a gas-fluidized bed. Mean and instantaneous values of the heat transfer coefficient were measured using various gases and solids at bed static pressures ranging from 0.03 to 1.48 MN/m². For all materials and conditions the heat transfer coefficient showed the same general dependence on gas flowrate. The heat transfer coefficient exhibited a maximum and the corresponding Nusselt number was related to the Galileo number and the Prandtl number of the gas by the equation Nu_max = 0.30 Ga^0.20 Pr^0.40 1<Ga<220From a trace of the element temperature against time, the corresponding instantaneous values of the heat transfer coefficient were calculated. At any gas flowrate the mean heat transfer coefficient could be expressed in terms of the coefficient at the incipient fluidization point and a term which was characteristic of the fluctuations.     

Experimental study of film condensation on horizontal grooved tubes

Experiments are reported for condensation of steam on horizontal grooved tubes of 1.9 cm diameter with 14.2 grooves/cm (36 tpi Standard American Screw Thread). Three tube wall materials were used: copper, brass and 70–30 cupronickel. The effect of noncondensable gas was investigated for steam-air mixtures in downward forced flow. Heat transfer enhancements of up to a factor of 5.5 were obtained with the brass tube. The deleterious effect of noncondensable gas proved to be no more serious than for smooth tubes. An overhead drip rate of up to ten equivalent tubes caused no significant decrease in heat transfer.     

氯乙烯预热器的优化选择

通过对预热器的各组分气体流量、气体温升等相关数据的测量,根据热量衡算计算出混合气体流速、总传热系数、热效率。比较分析了三者之间的关系,从而对预热器进行优化选择。     

Direct thermal drying of sludge using flue gas and its environmental benefits

This paper reported a sludge disposal technology that uses 100–200°C flue gas to dry sludge through a systematic analysis of the relationship between sludge drying rate/temperature and moisture content. Using this direct drying technology, the sludge drying capacity for the dryer tested can reach 86?tons?d^?1 at 160°C. The experimental results show this technology can also preserve 95% calorific value in the sludge, and remove 16–42% PM_2.5, 26–55% PM₁₀, and 7–25% SO₂ from the flue gas. The exhaust gas from the sludge dryer consists mainly of chain alkanes while benzenes only 9.65% when dried at 100°C.     

Bed-to-wall heat transfer characteristics in a circulating fluidized bed

The bed-to-wall heat transfer coefficients were measured in a circulating fluidized bed of FCC particles (d_p = 65 μm). The effects of gas velocity (1.0–4.0 m/s), solid circulation rate (10–50 kg/m²s) and particle suspension density (15–100 kg/m³) on the bed-to-wall heat transfer coefficient have been determined in a circulating fluidized bed (0.1 m-ID x 5.3 rn-high). The heat transfer coefficient strongly depends on particle suspension density, solid circulation rate, and gas velocity. The axial variation of heat transfer coefficients is a strong function of the axial solid holdup profile in the riser. The obtained heat transfer coefficient in terms of Nusselt number has been correlated with the pertinent dimensionless groups     

LNG绕管式换热器壳侧降膜沸腾数值模拟研究

使用ANSYS Fluent商用软件下的VOF模型,对天然气液化工艺中所用的绕管式换热器壳侧降膜沸腾过程进行了数值模拟。结果表明,当壳侧入口干度x ≤ 0.10时,VOF模型能较好地模拟壳侧沸腾换热现象,准确地预测出沸腾换热系数与冷剂质量流率间的关系。降膜流状态下,汽、液相流速较低,摩擦压降较小,换热管壁面基本被液膜覆盖,液相冷剂在受热壁面汽化后进入壳侧流道,流道内以汽相为主。壳侧既有竖直向下的主流运动,又有较为微弱的螺旋环流,同时大量的汽相将在壳侧顶部涡旋滞留。     

Experimental performance comparison of shell-side heat transfer for shell-and-tube heat exchangers with middle-overlapped helical baffles and segmental baffles

Presented in this paper are experimental test and comparison for several shell-and-tube heat exchangers, one with segmental baffles and four with helical baffles at helix angles of 20^°, 30^°, 40^° and 50^°, respectively. The results show that, based on the same shell-side flow rate, the heat transfer coefficient of the heat exchanger with helical baffles is lower than that of the heat exchanger with segmental baffles while the shell-side pressured drop of the former is even much lower than that of the later. Further enhancement techniques should be incorporated in order to enhance shell-side heat transfer based on the same flow rate. The comparison of heat transfer coefficient per unit pressure-drop (and pumping power) versus shell-side volume flow rate shows that (1) the heat exchanger with helical baffles have significant performance advantage over the heat exchanger with segmental baffles; (2) for the same shell inner diameter, the performance of heat exchanger with helical baffles with 30° helix angle is better than that of 20°, and the performance of 40° helix angle is better than that of 50° helix angle. The heat exchanger with helical baffles of 40° angle shows the best performance among the five heat exchangers tested.     

Experimental technique for kinetic study of hydrogenation of fatty acid methyl esters in vapor phase

A new approach to kinetic studies of fat hydrogenation is discussed. An experimental setup is described in detail. An example of reactor performance in hydrogenation of fatty acid methyl esters is given. aPresent address: AB Karlshamns Oljefabriker, S-292 00 Karlshamn, Sweden. Notation: c, outlet concentration, mol/m³ ; c_o, inlet concentration, mol/m³: c_b, concentration in bulk fluid, mol/m³ ; c_s concentration at catalyst surface, mol/m³ ; d,pore diameter, m; D_e, effective diffusivity, m²/s; E, activation energy of reaction, J/mol; h, heat transfer coefficient, J/m² s K; ΔH, heat of reaction, J/mol; k_c, mass transfer coefficient, m/s; p, partial pressure, Pa; p_o, saturated vapor pressure, Pa; qf, total flow to reactor, m³/s; q_rec, recycle flow, m³/s; R, observed reaction rate per unit particle volume, mol/s m³ ; R_g, gas constant, J/mol K; r, observed reaction rate per unit mass of catalyst, mol/kg s; r_p particle radius, m; T_b, temperature of gas in bulk flow, K; T_s, temperature of gas at catalyst surface, K;-v, molar volume, m³/mol; V, volume of catalyst bed, m³ ; W, catalyst mass, kg. Greek symbols: σ, surface tension, N/m; λ_e, effective thermal conductivity of catalyst particle, J/m s K.     

Effect of heat transfer conditions on the critical pressure of metal ignition in oxygen

Experimental data on the critical pressure of ignition of titanium alloy fragments in gaseous oxygen are analyzed. The fragments are obtained after fracture of alloy samples in the dynamic mode (p₂^*) and under natural convection conditions (p₁^*). The results are analyzed with allowance for the heat transfer coefficients from material ignition initiators under similar conditions. Based on the shape of the experimental thermograms of plate cooling, the coefficient of heat transfer from microcraters with a juvenile surface formed due to knockout of metal particles from the plate by the high-velocity flow is found: α₂ ≈ 11 kW(m² ·K). The value of α₂ is close to the value of this coefficient calculated with the use of the coefficient α₁ ≈ 5 kW/(m² · K) of heat transfer from titanium rod microfragments (with the size of the order of metal grains) formed during titanium rod fracture in oxygen under conditions of natural convection with allowance for the ratio p₂^*/p₁^*.     

An investigation of heat transfer from a vertical tube in a spouted bed

An experimental investigation on heat transfer from a vertical tube in a gas—solid spouted bed has been conducted. Values of heat transfer coefficients (h_s) have been found to vary between 95 and 230 W/m²°C. Maximum values of h_s are always at the spout axis and decrease monotonically towards the column wall. The drop is most important at the spout-annulus boundary. Most of the effects from different parameters can be explained by the flow patterns of the gas, although particle convection still contributes to some extent to the heat transfer rate. Equations correlating h_s in the spout and the annulus have been developed.     

曲面弓形折流板换热器壳程流体流动与传热

提出一种新型折流板--曲面弓形折流板,并构造曲面弓形折流板换热器,采用数值模拟和实验相结合的方法研究其壳程传热和流动阻力性能。在实验方面,设计了实验用曲面弓形折流板和普通弓形折流板换热器试样,其中换热器管束采用可拆连接形式,以考察不同折流板结构和板间距的影响。通过改变管程及壳程流量和管程流体进口温度,获得了大量对应于不同折流板结构的壳程压力降和传热系数实验数据。在模拟方面,利用Fluent软件建立了曲面弓形折流板换热器和普通弓形折流板换热器流体数值分析模型,得到了壳程流体流场分布及壳程压力降和传热系数。结果发现,在相同结构参数和流动条件下,曲面弓形折流板换热器壳程压力降比普通弓形折流板换热器降低9%～24%,而壳程传热系数比普通弓形折流板换热器提高3%～11%。