河网水力数值模拟的追赶法模型的应用

以Saint-Venant方程组为基础,介绍一种线性代数方程组的追赶法模型在汀江上杭河段洪水演算中应用,获得了满意成果。该算法简单明了,易编通用程序,舍入误差小和数值稳定性好。     

Modeling the thawing of frozen foods using air impingement technology

With the continual growth in the use of frozen foods both in retail and in food service, there is a need to develop improved thawing methods. Current methods are often undesirably slow (still air) or are very expensive and cause uneven thawing (microwave). Air impingement technology is one possible method to improve the thawing of frozen foods. The objectives of this research were to develop a two-dimensional model for air impingement thawing frozen foods and to verify the model experimentally. Frozen products were thawed using a laboratory impingement system with a single impingement jet. A simulated meat product (Tylose gel) was used as the test material. Thawing of a Tylose disk (12.7 cm diameter, 1.98 cm thickness) with air at 6 °C without impingement required more than 12 h, while thawing under a single impingement jet took less than 3 h, over four times faster. Results from the finite difference model gave good agreement with experimental data. Moisture loss during thawing was typically over-predicted because moisture gain due to condensation was not modeled.     

高分辨谐波探测技术在反射光谱中的应用

在对激光器采用电压调制的情况下进行了Cs原子D2线中6S1／2（F＝4）→6P3/2（F=5）电子跃迁反射光谱的二次、四次和六次谐波探测,并通过数值分析得到了低压气体在谐波探测下调制展宽与调制系数的关系,实验结果与理论分析吻合较好,结果表明,选取合适的调制度及高次谐波有利于提高信号的分辨率。     

Effects of shapes of electrodes on freezing of supercooled water in electric freeze control

In order to clarify the effects of electric charge on freezing of supercooled water, experiments were carried out. Two kinds of shapes were used for tips of electrodes. One was the sharp end surface. The other was the flat end surface. Aluminum was selected as the material. Water sample was kept in a test tube and cooled down at a constant cooling rate. When the water sample was maintained under a supercooling state, an electric charge was applied to the water sample with a small electric current. The degree of supercooling and the electric current were measured, continuously. Then the degree of supercooling at freezing was determined. It was found that the effects of electric charge were distinct according to the shapes of electrodes. The degree of supercooling at freezing was the lower in the case of flat end surface of the anode than that in the case of sharp end surface of the anode.     

The effect of salt concentration on the freezing point of meat simulants

Accurate data on the initial freezing point of cured meat is required to predict freezing rates or identify optimal slicing temperatures. However, little data was found in the literature. Experiments were therefore carried out using the ‘Karlsruhe test substance’ (‘Tylose’) with varying salt concentrations as a cured meat substitute. Initial freezing points were −1.4, −3.1, −4.1, −5.2 and −6.3 °C at salt contents of 0.5, 2, 3, 4 and 5 kg salt/100 kg sample, respectively. These values were within ±0.5 °C of published values for cured pork and within ±0.9 °C of theoretical predictions. Modifying the salt content of Tylose is therefore a simple way of determining the initial freezing point of cured lean meats, and Tylose modified in this way can be used to simulate the freezing of cured meat.     

Modeling of frosting behavior on a cold plate

This paper proposes a mathematical model to predict the frost properties and heat and mass transfer within the frost layer formed on a cold plate. Laminar flow equations for moist air and empirical correlations for local frost properties are employed to predict the frost layer growth. Correlations for local frost density and effective thermal conductivity of the frost layer, derived from various experimental data, are expressed as a function of the various frosting parameters: the Reynolds number, frost surface temperature, absolute humidity and temperature of the moist air, cooling plate temperature, and frost density. The numerical results are compared with experimental data to validate the proposed model, and those agree well with the experimental data within a maximum error of 10%. Heat and mass transfer coefficients obtained from the numerical analyses are also presented. The results show that the model for the frost growth using the correlation of the heat transfer coefficient without considering the air flow has a limitation in its application.     

Cellular cryobiology: thermodynamic and mechanical effects

Several physical stresses kill cells at low temperatures. Intracellular ice is usually fatal, so survival of freezing temperatures involves combinations of dehydration, freezing point depression, supercooling and intracellular vitrification. Artificial cryopreservation achieves intracellular vitrification with rapid cooling, modest osmotic contraction and, often, added cryoprotectants. High warming rates are required to avoid crystallization during warming. Environmental cooling is much slower and temperatures less cold, but environmental freezing damage is important ecologically and agronomically. For modest sub-freezing temperatures, supercooling sometimes allows survival. At lower temperatures, extracellular water usually freezes and cells may suffer large osmotic contractions. This contraction concentrates solutes and thus assists vitrification, but is not necessarily reversible: the rapid osmotic expansion during thawing may rupture membranes. Further, membranes and other ultrastructural elements may be damaged by the large, anisotropic mechanical stresses produced when their surfaces interact via hydration forces. Solutes reduce these stresses by osmotic, volumetric and other effects.     

Experimental correlation of pool boiling heat transfer for HFC134a on enhanced tubes: Turbo-E

The objectives of this paper are to study the heat transfer characteristics for enhanced surface tubes in the pool boiling and to provide a guideline for the design conditions for the evaporator using HFC134a. The shape of tube surfaces, the wall superheat, and the saturation temperature are considered as the key parameters. Copper tubes (d_o = 19.05 mm) are treated with different helix angles and the saturation temperatures are controlled from 3 to 16 °C. It is found that the pool boiling heat transfer coefficient decreases with increasing the wall superheat. It is also found that boiling heat transfer coefficients for Turbo-II and Turbo-III are 1.5–3.0 times and 1.2–2.0 times higher than that for Turbo-I without the helix angle, respectively. The higher heat transfer performance from Turbo-II and Turbo-III can be explained by the “bubble detention” phenomenon on the surface without the helix angle for the Turbo-I. The experimental correlations for the pool boiling heat transfer on the present enhanced tubes without (Type I) and with the helix angle (Type II and Type III) are developed with the error bands of ±30%, respectively.     

Condensation of hydrocarbons – A review

Hydrocarbons are one of the candidates for refrigerants of next generation heat pump and refrigeration systems. Although the hydrocarbons have superior thermophysical properties as a refrigerant to fluorocarbons and are widely used in domestic refrigerators, their flammability prevents their wide application to larger systems, such as residential and packaged air conditioners, car air conditioners, heat pumps, etc. In this paper, recent studies on condensation of hydrocarbons are reviewed since it is one of the key technologies. For in-tube condensation, heat transfer coefficients of smooth tubes are correlated well with previously proposed equation developed with data of the fluorocarbons. On the other hand, few data are available for enhanced tubes. In the case of condensation on a horizontal tube, heat transfer of smooth tube is explained well by the Nusselt theory. However, different heat transfer characteristics appear for enhanced tubes. Since blended hydrocarbons would be used to obtain suitable properties, models of mixed vapor condensation are also reviewed.