Hello Piyush Uniyal

Hello you need to know first the kind of heat exchanger you will use to the heat transfer, then you need to know the initial and final temperature of the air to know, How many is the mass range an Reynold number. Remember this:

Qtranfer=moCP(Tf-Ti);  So:

Qtranfer= Heat transfer to the air.

mo= Flow rate, kg/s or lb/s

Tf= Final Temperature

Ti= initial Temperature; air = Ambiental Temperature. 

So the Reynolds Number depends of:

Re=vL/∂

v= Velocity of the principal mass,

L= hidráulica Diameter or length,

∂= kynetic viscosity. 

The Reynold define if the flow are in laminar or turbulence flow, but the range of this depend if you have flow on a plate or in a pipe or out a pipe. 

So you can try to do your experiments with another parameters too:

Prandtl,  Nusselt and Reynolds Number.  So this number are variable for the temperature difference of the initial and the final flow mass. but you can find a mathematical correlation to find then: Try to change the Prandtl and the Reynolds to demonstrate your Solar air heater. 

I hope help you something. 

best regard

Fernando gave you a good kickstart , I think if you may want to explore characteristics of different designs to give you an idea of the operational ranges. Good sources:

- Hesselgreaves, J.E.. (2001). Compact Heat Exchangers - Selection, Design, and Operation. Elsevier.

- Serth, Robert W. Lestina, Thomas G.. (2014). Process Heat Transfer - Principles, Applications and Rules of Thumb (2nd Edition). Elsevier.

- Branan, Carl R.. (2005). Rules of Thumb for Chemical Engineers - A Manual of Quick, Accurate Solutions to Everyday Process Engineering Problems (4th Edition). Elsevier.

The range of mass flow rates and Reynolds Numbers for your study depends on whether the configuration of your device is active or passive.

If you are doing experimental work, the only you have to do is set up your experiment with the specific temperatura sensors to detect the changes and with all the advise given by Fernando Juan Proano, your problem should be solved

Hi

You may divide the problem into two schemes

For Mass flow rate (Energy Conservation Principle)

1. First you need to clearly define your experimental model and precisely define the most significant modes of Heat Transfer (Conduction or Radiation). 

2. Evaluate the Total Energy from your heater source taking note of the most significant heat transfers modes.

3. Obtain a relevant mathematical equation which captures energy difference for the fluid flowing over the heater surface taking note of the most significant HT modes.

4. Equate (2) and (3)  and solve for the mass flow rate term.

For Reynolds No 

5. Set up a continuity equation for your model (Mass conservation) relating mass flow-rate and volume flow-rate and evaluate volume flow rate ( You may obtain fluid density using true-mean value of function over a range or look it up at an average temp )

6. For a cross-section of interest and at distance from the leading edge, you may now evaluate fluid Velocity Distribution and consequently fluid cross-sectional mean velocity.

7. Using results from (5) and (6) you may reasonably estimate your Re value .

Thanks.