Are there recommended (standardized) definition of the criteria for achiving stationary conditions in a laboratory measurement of heat transfer in heat exchangers? I found in the literature some recommendations in terms of permissible deviation of the media temperature at the inlet to the heat exchanger. There are also some recommendations regarding the differences in heat exchanged on both sides of the media. But the values are quite different. Also mentioned is the permissible change in the amount of fluid during the measurement?
Hello Mr. Mladen Bošnjaković
Your question is quite important for experimentalist in thermal sciences. As a fluid mechanics researcher, I don‘t have many references of heat exchange experiments. However, I can think of some ideas around your question based on the phenomenology.
- experiments are often carried to evaluate specific parameters of a target problem we‘d want to understand or gather information about. Secondary phenomena can usually appear;
- since your experiment have particular dimensions, geometry, etc., I‘d say that you should find the time scales (dimensional analysis) for the phenomena you‘d want to monitor, and compare it to secondary variations of measured parameters over time. At this point you could establish a rate of properties variations and consider them stationary or not.
- if your boundary conditions are fixed, your control volume is adiabatic, and your flow is stationary, you can admit stationary heat exchange at some point, and find the rate of properties variations which would be related to your stationary heat exchange. Could those variations be somehow related to turbulence dissipation?
- once you have defined this, what rests is your phenomena, stationary. Any further variation over time, consider example time evolution of boundary conditions, would lead To a time-dependant measure.
I hope I was able to to give some ideas to pursue your problem.
Guilherme Fiorot
OK, thank you.
Some criteria in literature are:
a) minimum ten minute sampling time interval (steady state)
b) exchanged heat variation in regards to average value during sampling time not greater than 5% (8% ?, 10?)
c) heat rate imbalance (between the water and air sides) should be no more than 5% (8% ?)
d) variation of inlet temperature of air and water not greater than 1 K
e) The biggest permissible change in mass flow of water and air is
Experiment is performed. Sampling time is 60 min. For analysis is taken 30 minutes (every second - 1800 valid data).
Temperature variation for air and water are within 1 K.
All other mentioned criteria is fulffield.
Temperature of air in inlet is about 20 °C, and water in inlet is about 80 °C. Air at outlet is in range 30 -40 °C, and water about 75 °C.
For your temperature ranges, the variation for air should be within 0.5 K while variation for water can be within 1 K.
Please,
explain why, and where is it stated (book, article, standard ...)
You can find it in many experimental published articles. If you want, I can send you samples of that articles.
Surely 1k is a bit high, does it not represent 20% of the 5 Celsius of the water, whereas .5k on say 15 Celsius represents 3% variation for the air?
dear, Mr. Shedid,
I check my data for air. Variation is within 0,4 K. This is according your proposal. I will take 0,5 K as proposal in my work ...
Thanks for your proposal ...
I found ASME standard:
Air Cooled Heat. Exchangers - ASME PTC 30-1991-PERFORMANCE
TEST CODES
It propose:
All instrument readings requirebdy this Code shall be taken after reaching steady-state process conditions.
- The air flow shall be within +-1O percent of the design value.
- Procêss fluid flow rate shall be within +-15 percent of the design value, and shall not vary by more than 5 percent during the test.
The process fluid temperature range shall be.hall be within +- 10 °F of the design values, and shall not vary by more than 4°F during the test.
- The process fluid temperature range shall be within +- 10 % of the design value, and shall not vary by more than 5 % during the test.
- The process fluid inlet pressure shall not vary by more than 10 % during the test
- heat balance discrepancy not greater than 15 %
I think that the objective of the measurement (test) should guide you:
The requirement in item 3 will provide you with what accuracy you need to measure, including biases, process variations, conceptual errors, etc. (Then you will need to set up an "uncertainty budget" and make choices on instrumentation accuracies and other experimental equipment in order to realise the measurement setup.)
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