Learning Forum

[sumeet94]

Hello,

I am solving parabolic trough receiver tube (PTRT) in Ansys 2020 R2 (Fluent solver) . Basically in the PTRT, absorber tube is enclosed within the glass tube with having some annulus space between them as shown in Fig. 1.

Fig. 1. Cross-sectional view of parabolic trough receiver tube (PTRT)

This annulus space is kept as vacuum condition to prevent the heat loss from conduction and convection mode except radiation mode.To analyze this heat loss, I am using surface-to-surface (S2S) radiation model, as this model is suitable when there is no participating medium exist.  The absorber tube receives non-uniform solar flux distribution across its absorber tube outer wall. The annulus space is considered as fluid domain filled with air, however, I have fixed all the flow variables as zero except temperature in the fixed value tab under cell zone condition. Also, to keep vacuum condition in this annulus space/domain, the pressure value of less than 0.01 Pa (approximately vacuum condition) is patched for this domain under patch tab after initialization of the problem. The working fluid is air, absorber tube material is stainless steel, and the glass cover material is borosilicate glass. The boundary conditions are given as, mass flow rate and temperature inlet at inlet section, pressure outlet as outlet section, absorber tube inner wall as no-slip boundary condion and interface condition, absorber tube outer wall as non-uniform heat flux boundary condition and radiation, glass cover inner wall as radiation and coupled condition, glass cover outlet wall as mixed boundary condition (convection and radiation), and all the sides are kept as insulated.

The problem in my result is that the temperature distribution is not appropriate as the majority of heat flow outside, and makes the glass cover temperature as very higher value (temperature range 411 K to 820 K), as shown in Fig. 2 (d), which is not practically possible. In realistic situation. this glass cover is proposed to have close to ambient temperature while the absorber tube should have higher temperature because of non-uniform flux condition. The temperature contours for different domains of PTRT is shown in Fig. 2.

(a)

(b)

(c)

(d) 

 Fig. 2. Temperature distribution of (a) PTRT tube at mid-section (b) annulus space (c) absorber tube and glass cover (d) glass cover.

Can anyone please help me that -

(1) Why most of the heat flows outward as shown in Fig. 2 (a) and the feasible temperature is not coming for PTRT ?

(2) By using patch option, the vacuum condition exist or not inside the annulus domain ? However, the pressure contour for this domain shows that pressure is zero, as shown in Fig. 3.

 

Fig.3. Pressure contour of PTRT at mid-section.

Thank you in advance, please help me in this.

[Rob]

What material density are you using for the gas regions?  

[sumeet94]

Air density of 1.1614 kg/m^3 for both fluid region and annulus space region

 

[Rob]

OK, and how would that represent a different mass of material and cause buoyant flow in the system? 

[sumeet94]

Sir, previously I calculated this annulus region air density using ideal gas equation to create buoyance effect in this region. However, the problem remains the same, so, I kept density as same value for both the fluid and annulus domain.

 

[Rob]

Remember most CFD codes (including Fluent) require the continuum assumption to be valid (ie no vacuums). Otherwise, with ideal gas you should be OK.