ATOM INDONESIA
Author's Responses
Article : #497
Name of All Authors :
Article Title : An Experimental Analysis on Nusselt Number of Natural Circulation Flow in Transient Condition Based on the Height Differences between Heater and Cooler
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Line # Referee’s Comments Author's Responses (1) 2017 Author's Responses (2) 2018
1. It is not clear how the experiment and its results can be applied to study the thermal hydraulic behavior of passive systems stated in the Introduction, such as PRHRS.
2. How the temperature, pressure, elevation difference between heat source and heat sink covered in the experiment can be applied to those systems via dimensional analysis approach?
As described in the introduction, that this research is focused on understanding the nature of thermal hydraulics based on temperature behavior due to changes in geometry. This study was conducted to understand natural circulation phenomenon in transient conditions. It more work is needed for comparison with other researchers with more variation in geometrical and thermal parameters using advanced experiment facility.
1. Yes, the statement was not clear and author changes from the lines No.62-65 into new statement: “Some researchers have investigated natural circulation phenomenon since several decades ago, the research aimed to evolve the reactor safety system by implementing they study into a passive residual heat removal system (PRHRS)” in new line 62-67.
2. In principle, the flow of natural circulation is affected by changes in thermal hydraulics parameters (temperature and pressure), geometry (total length of pipe, pipe diameter, and height difference between heat source and coolant source). Includes pressure loss factors caused by material properties and piping components. The natural circulation flow represents as the variable of the Reynold number in a non- dimensional number based on the
kinematic aspect. In thermal hydraulivs terms, one of variable in Nusselt number is Reynold number (also Prandtl). Thus, the temperature, pressure and elevation difference between heat source and heat sink can be applied based on the dimensionless number to the system.
3. The authors stated that their work was aimed at the unsteady-state (transient) natural circulation, and there is no previous works on transient natural circulation stated in the Introduction. It is not convincing:
(1)There are a lot of works already done in the transient natural circulation field; the authors should mention those works.
(2)The authors should show in terms of equations, mathematical models or correlations which show the differences between “steady-state” natural circulation and
“transient” natural circulation the authors claimed.
(3)In my opinion, the authors measured the correlation under a quasi steady-state of natural circulation, not real transient one, unless the authors are able to show the time scales for transient natural circulation are much longer than the experiment time scales.
The intended transient condition is the absence of heat input into the system during the cooling process. Obviously with the experimental conditions using the initial temperature of 90 ° C for water, the rate of temperature decrease was very fast.
Meanwhile, the transient condition by other researchers is done by decreasing the heat input to the system gradually during the cooling process occurs.
(1). -
(2). Which model? This work was done in transient condition.
(3). This is wrong statement; the experiment was done in transient condition.
4. Reynolds Number is the main and important component to derive the Nusselt number correlation, however, the fluid velocity was not directly measured, and consequently the Reynolds Number was not derived from the measured fluid velocity. This is a significant weakness of the experiment. If the authors insisted to use the approach, then they should discussed (1) the error of the Re number they
Again as described in question 1, that this research is focused on understanding the nature of thermal hydraulics based on temperature behavior due to changes in geometry. It is clear that the resulting flow velocity results from the calculation using equation (1). With, speed as a function of temperature and geometry changes, such as height difference. However, further research
obtained, (2) whether it can be applied to
“transient” natural circulation they claimed from this experiment will use flow rate measurements using the new experimental facility.
5. The Nu correlation was fitted by using 3 measurement data and compared to the one of Yu. There are some logical problem with this approach:
(1) If the authors’ measurement data is aimed at
“transient” natural circulation with no previous work available why they compared with other
“previous work” correlation ?
(2) Is it acceptable to use equation (7) for
“transient” natural circulation ? See comment no 2 above.
The fitted data curve is performed by taking the average value of the Nusselt number on the descending condition with a very small gradient (see Fig.7). Thus, the profile of Nusselt number curve with the smallest gradient (considered stable) is compared with the Yu equation which decreases the Nusselt number with a small gradient as well. The similarity with the computer simulation done by Yu is the pressure condition below 5 bar. In conclusion, comparison with other researchers work was done with a condition that is almost similar.
Equation (7) is a simplified equation as a derivative to indicate the Nusselt number as a function of height differences.
Experimental results for the average Nusselt number are used to show that equation (7) can be applied in the results of this experiment.
The fitted data curve is performed by taking the average value of the Nusselt number on the descending condition with a very small gradient (see Fig.7). Thus, the profile of Nusselt number curve with the smallest gradient (considered stable) is compared with the Yu equation which decreases the Nusselt number with a small gradient as well. The similarity with the computer simulation done by Yu is the pressure condition below 5 bar. In conclusion, comparison with other researchers work was done with a condition that is almost similar.
Equation (7) is a simplified equation as a derivative to indicate the Nusselt number as a function of height differences.
Experimental results for the average Nusselt number are used to show that equation (7) can be applied in the results of this experiment. (the comment is the same as previous)
6. The measured data for elevation difference of 30 cm may not be valid (too small) because of the dimension of heat source and heat sink.
This may induce difficulty in the fitting process shown in Figure 8.
Validation cannot be indicated by the magnitude of the 30 cm height difference only. But, the validation of this research based on the changes of thermal and geometrical variables of as a thermal hydraulics functions, such as Reynold and Nusselt number. The point is, the indication of temperature and height differences changes were indicated by the calculation of natural circulation flow rate, Reynold number and Nusselt number also changed and have a certain pattern. Especially in
transient conditions.
