Transient cooling

Figure 2.32 Dependence of the dimensionless critical heat flux on the contact angle during steady-state heating and transient cooling. (From Liaw and Dhir, 1986. Copyright 1986 by Hemisphere Publishing Corp., Washington, DC. Reprinted with permission.)...

It is thus clearly demonstrated that the accurate predictions of CHF (or dryout) delay and the existence of transition boiling are very important in the evaluation of a maximum clad temperature in this type of accident. The test results of Tong et al. (1965, 1967a) and of Cermak et al. (1970) indicate the validity of using the steady-state CHF correlation to predict the CHF during a PWR transient cooling or blowdown. 

Kaka , S. and Cotta, R.M. (1993), Experimental and Theoretical Investigation on Transient Cooling of Electronic Systems, Proc. of the NATO Advanced Study Institute on Cooling of Electronic Systems, Invited Lecture, NATO ASI Series E Applied Sciences, Vol. 258, pp. 239-275, Turkey, June/July. 

Steady-state temperature fields are independent of time, and are the end state of a transient cooling or heating process. It is then valid that dd/dt = 0, from... 

The transient thermal stress without any crack present, at the center of the surface of the specimen corresponding to the transient temperature distribution is shown in Figure 5. Initially the surface is in residual compression. As the heating initiates, the compression increases, peaks and decreases to come to some steady-state value. The subsequent transient cooling causes a transient tensile stress on the surface. 

A typical transient cooling curve for the various sections of a bottle is shown in Figure 3. A typical plot of the resulting (non-uniform) stress distributions are shown in Figure 4-6. The stress values used to create these plots were measured in an immersion polariscope. From these measured stress maps, and knowing the wall thickness, it is possible to estimate the heat transfer coefficients. These numbers vary over the surface of the bottle but typically range between 150 W/m -°K to 300 W/m -°K. 

Transient cooling rates were measured to study the effect of adding droplets. The cooling time is defined as the time to cool the test plate from 500 C to 35°C. These measurements were made with both the single-phase and two-phase flows. A shutter was used to close the nozzle exit to avoid any flow impingement on tiie test plate. The test plate was heated to 500°C with a constant power input of 310 Watts. The same power input was used for each experiment to msure that, once the plate reached the desired temperature, the insu tion and the heater reached the same conditions for every test Once the piste was heated up to 500°C, the shutter was opened and the heater was turned off to start the cooling process. 

Fig. 13 Single-phase and two-phase flows transient cooling.

An experimental stu% was performed to determine fee effect of small water droplets incorpmated into a single-phase stagnation-point flow on heat transfer. Steady state and transient cooling heat transfo erqieriments were carried out to analyze fee characteristics of single-phase and two-phase flows. PIV measurements were made for different air velocities to determine the characteristics of the flow. Water droplets size distribution was also measured. The following conclusions can be made fiom this study ... 

Dissolve 1.5 g of [Co(NH3)sCl]Cl2 in 20 cm of 2 M NH3 solution. Heat in a water bath until the salt dissolves. Cool and acidify to pH4 with 4M HCl. Add 2g NaNOj and heat gently until the red precipitate first formed has dissolved. (The precipitate may only appear transiently). Cool and add carefully 20 cm cone. HCl. Cool in ice and filter off the yellow brown crystals and wash with alcohol. Dry by continued suction while pressing between filter paper. Record your yield and calculate the % yield. 

Ryan, P. J. and Harleman, D. R. F. 1973. An analytical and experimental study of transient cooling pond behavior, T. R. 161, R. M. Parsons Laboratory, Massachusetts Institute of Technology, Cambridge, MA. 

Operation of a reactor in steady state or under transient conditions is governed by the mode of heat transfer, which varies with the coolant type and behavior within fuel assembHes (30). QuaHtative understanding of the different regimes using water cooling can be gained by examining heat flux, q, as a function of the difference in temperature between a heated surface and the saturation temperature of water (Eig. 1). 

Maintenance of isothermal conditions requires special care. Temperature differences should be minimised and heat-transfer coefficients and surface areas maximized. Electric heaters, steam jackets, or molten salt baths are often used for such purposes. Separate heating or cooling circuits and controls are used with inlet and oudet lines to minimize end effects. Pressure or thermal transients can result in longer Hved transients in the individual catalyst pellets, because concentration and temperature gradients within catalyst pores adjust slowly. 

Because the system likely is nonisothermal, the analysis of a closed-desiccant system requites knowledge of the temperature of the desiccant as well as the dew point (ice point) or water concentration (partial pressure) specification. Indeed, the whole system may undergo periodic temperature transients that may compHcate the analysis. Eor example, in dual-pane windows the desiccant temperature is approximately the average of the indoor and outdoor temperatures after a night of cooling. However, after a day in the sun, the desiccant temperature becomes much warmer than the outdoor temperature. When the sun sets, the outdoor pane cools quickly while the desiccant is still quite warm. The appropriate desiccant for such an appHcation must have sufficient water capacity and produce satisfactory dew points at the highest temperatures experienced by the desiccant. 

Slime masses or any biofilm may substantially reduce heat transfer and increase flow resistance. The thermal conductivity of a biofilm and water are identical (Table 6.1). For a 0.004-in. (lOO-pm)-thick biofilm, the thermal conductivity is only about one-fourth as great as for calcium carbonate and only about half that of analcite. In critical cooling applications such as continuous caster molds and blast furnace tuyeres, decreased thermal conductivity may lead to large transient thermal stresses. Such stresses can produce corrosion-fatigue cracking. Increased scaling and disastrous process failures may also occur if heat transfer is materially reduced. 

Function event trees include primarily the engineered safety features of the plant, but other systems provide necessary support functions. For example, electric power system failure amid reduce the effectiveness of the RCS heat-removal function after a transient or small UJ( A. Therefore, EP should be included among the systems that perform this safety function. Siipfiort systems such as component-cooling water and electric power do not perform safety functions directly. However, they significantly contribute to the unavailability of a system or group of systems that perform safety functions. It is necessary, therefore, to identify support systems for each frontline ssstcm and include them in the system analysis. 

Nuclear power plant systems may be classified as "Frontline" and "Support. . iccurding to their. service in an accident. Frontline systems are the engineered safety systems that deal directly with an accident. Support systems support the frontline systems. Accident initiators are broadly grouped as loss of cooling accidents (LOCAs) or transients. In a LOCA, water cooling the reactor is lost by failure of the cooling envelope. These are typically classified as small-small (SSLOCA), smalt (SLOCA), medium (MLOCA) and large (LLOCA). 

A transient, is a passing event which may upset the reactor operation but does not physically damage the primary cooling envelope. Table 6.1-1 lists PWR transient initiating events that ha c been used in PRA preparation. Typical frontline systems that mitigate LOCAs and transients for a PWR are presented in Table 6.1-2. The frontline systems must be supported by support systems interactions between both are presented in Table 6.1 -3 for ANO-1 (Arkansas Nuclear Unit 1). 

---