AN ANALYSIS OF PHASE DISTRIBUTION AND TURBULENCE IN DISPERSED PARTICLE/LIQUID FLOWS

An analysis of dilute, turbulent particle/liquid two-phase flow is presented. The three-dimensional conservation equations that govern turbulent motion in solid/fluid flows are derived using ensemble averaging, and the unknown terms in these equations are constituted to achieve closure. These closure terms include the shear stress due to interparticle collisions, the corresponding terms in the Reynolds stress equation, the force and dissipation due to particle-wall collisions, and the interfacial work due to particle/turbulent eddy interaction. The resultant two-fluid model was then evaluated using a computational fluid dynamic (CFD) solver and the predictions were compared with experimental data. Good agreement was observed for a variety of flow conditions.     

A NEW TOOL FOR NUCLEATE BOILING RESEARCH

A new tool for the study of nucleate boiling heat transfer has been developed. Photographic etching was shown to be effective in the manufacture of copper lest surfaces containing precise arrays of artificial pits, with densities as high as 2742 pits/cm².

The pits were shallow segments of spheres, with depth: diameter ratios on the order of 0.1. Pit diameters were on the order of 0.003 cm. Data showed that the pits significantly affected the characteristic boiling curves and critical fluxes for the surfaces studied while boiling Freon 113 at one atmosphere pressure.

Photoetching can be used in the future to produce a wide variety of test surface textures or site arrays. Careful control of such surface conditions may well lead to a better understanding of the nucleate boiling phenomena.     

EFFECT OF CONTAMINANTS ON CRITICAL HEAT FLUX AT LOW PRESSURES

Critical heat flux (CHF) tests were performed to evaluate the effect of dissolved, nonreactive contaminants on low-pressure industrial boilers. These tests were conducted on a 2.38-inch (60.5 mm) I.D. vertical smooth bore tube with nonuniform circumferential heating at pressures between 100 and 500 psia (0.69 and 3.45 MPa). Tests were performed under two water chemistry conditions: clean (less than 1.6 ppm total dissolved solids) and contaminated (greater than 2000 ppm TDS). With all other operating parameters held constant, the following effects of contamination on the steam quality (X) at CHF were determined at the pressures indicated:

At 100 psia (0.69 MPa) X_clean < X_cont

At 300 psia (2.1 MPa) X_clean?X_cont

At 500 psia (3.5 MPa) X_clean > X_cont

The effect of contamination on CHF was found to be a function of pressure, initial contaminant concentration, and the relative steam quality at which CHF conditions occur. These results are compared to data available in the open literature where X_clean is always greater than X_cont. A method for correlating these data is also illustrated.     

A NONDESTRUCTIVE METHOD TO MEASURE RESIDUAL ADSORPTION CAPACITY OF CHARCOAL FILTERS

High surface area charcoal bed filters have been used for over a half a century to adsorb undesirable vapors from gas streams. One problem encountered when using these niter beds is that there is presently no simple, reliable, nondestructive method to measure their Residual Adsorption Capacity, RAC. This is particularly critical in situations where harmful vapors are being adsorbed.

An investigation has been underway to use pulses of weakly adsorbed gases such as ethane and/or methane to measure RAC. The hypothesis being that these weakly adsorbed gases will “count” unoccupied adsorption sites. In the present study, dimethyl methylphosphonate (DMMP) was used to “irreversibly” occupy available siles to various extents on different niters. The Reduced Retention Time. θ, (the ratio of the adjusted retention time to the space time) and the Resolution (R) between peaks of methane and ethane were found to correlate to RAC under dry and wet (humid) conditions.     

粗纤维素酶液对微晶纤维素酶解的影响因素

栗生灰黑孔菌多被用于产漆酶的研究,很少有利用其纤维素酶的报道。为了降低在纤维素水解中纤维素酶的使用成本,利用栗生灰黑孔菌发酵制备的粗纤维素酶液,以微晶纤维素为底物模型,研究粗纤维素酶液水解微晶纤维素的最佳pH、温度和最佳表面活性剂助剂种类及浓度,并对不同表面活性剂存在条件下的纤维素酶解动力学、紫外和荧光光谱进行了研究。结果表明,粗纤维素酶水解微晶纤维素的最佳条件为pH 4.8,温度50℃,最佳表面活性剂助剂为吐温80,添加剂量为1.12mg/g底物;吐温80的添加可提高粗纤维素酶解的最大反应速度常数V_max,降低米氏常数K_m;表面活性剂改变了纤维素酶的紫外和荧光最大吸收峰,酰胺Ⅰ带和酰胺Ⅲ带的谱峰,可能通过与纤维素酶中的氨基酸残基发生反应影响了纤维素酶的结构,进而影响了微晶纤维素的水解反应。该研究为进一步降低纤维素水解成本提供了理论指导。     

Convection Heat Transfer Enhancement on Recirculating Flows in a Backward Facing Step: The Effects of a Small Square Turbulence Promoter

This work addresses the numerical simulation of incompressible turbulent recirculating channel flows in a backward-facing step. The effects of a small square turbulence promoter on convection heat transfer are evaluated through a parametric study. The governing equations comprise the time-averaged mass, linear momentum, and energy conservation principles in conjunction with the two-equation k–epsilon turbulence model. The study is focused on the assessment of the local and global Nusselt numbers at the channel stepped wall. The main results indicate that a maximum increment around 15% on the average Nusselt number can be achieved by using a small turbulence promoter to disturb the flow. Furthermore, it was found that the peak of the local Nusselt number on the stepped wall is located in the region where the turbulent diffusion is maximum in the near wall region.     

RHEOLOGY OF SUCROSE-CMC MODEL SOLUTION

The rheological behavior of an aqueous solution composed of 65% sucrose and 0.1% CMC was experimentally determined from -10C to 50C using a rotational viscometer equipped with coaxial cylinders. The model solution behaved as pseudoplastic fluid, represented by the power-law model. Flow behavior and consistency indices determined at -10.1, 0.5, 9.8, 19.0, 30.1 and 50. OC, showed values between 0.79 to 0.86 and 0.15 to 10.22 Pa. sⁿ, respectively. The effect of temperature on the rheological parameters was represented by Arrhenius-type relationships, with R²≥ 0.97. They were compared to the rheological behavior of a concentrated orange juice with 65° Brix and 7% pulp content. The comparison of separate rheograms for both fluids showed that the model solution can be used to represent concentrated orange juice.