Dear Mohammed,

the first approach can be considered as complete boiling thus it is the closest to real applications. if you have access to local information like the void fraction or local wall temperature (via infrared metrology of thermocouples) thus you can deduce many important parameters.

the second approach, I consider it a replacement of the real behavior of boiling due to the lack of instrumentation or to the complex geometry. otherwise, this approach allows the investigation of the complete flow regimes that you might not have in the real application.

in my opinion it will depend on the material that already exist in your laboratory and on the budget for buying new material. it will depend on the boiling procedure: is it by using a heating wire or by using water (tube in tube) or...?

thus you can obtain accurate experimental data in both approaches depending on the precision of the material used.

it might be helpful to do a literature review on boiling in vertical tubes and for each experimental study you can identify the experimental setup (1st or 2nd approach), metrology used with its precision, parameters obtained/measured/calculated with their uncertainties.... this is what I have done during my Ph.D. and it was really helpful to understand and decide later on which method to use.

I hope this is helpful to you.

good luck.

I agree with dr. Sarraf - the method you choose basically depends on equipment and funding availability. There are pros and cons to both methods and you should study the literature in order to appreciate the possibilities and limitations of both methods. You should also pay attention to the method of applying heat to your tube. Stainless steel has low thermal conductivity, hence using a heating wire wrapped around the tube may introduce an uneven heat flux, which may give inaccurate results in the subcooled/nucleate boiling dominated regions. In earlier days, direct resistance heating of the tube was often used, but this is expensive...

If you further want to explore the dryout point, you should add to your list of potential uncertainties that the flow history may also play a role. This is linked to the entrainment of droplets in the vapor core. Best of luck in your research.

thanks so much for your explanation really I appreciate that and as you suggested I did a comprehensive review regard this matter and really this was very helpful to collect valuable info and I discussed with my supervisor to put every thing clearly especially about the funding. thanks again for your response and have a nice time