Advanced Applications of Supercritical Fluids in Energy Systems by Lin Chen Yuhiro Iwamoto

Author:Lin Chen,Yuhiro Iwamoto
Language: eng
Format: epub
Publisher: IGI Global

Niu et al., 2011.

Figure 16. Test section

Niu et al., 2011.

However, the difficulty arises to the direct measurement of the evacuated solar collector, due to the complex channel geometry of the collector and system path, where CO2 is enclosed in the pressurized state.

In order to obtain fundamental data, the test section of a simple geometry is used instead of evacuated solar collector to simulate the supercritical CO2 flow experimentally, the schematic diagram of the experimental facility is shown in Figure 15. The detailed structure of the test section ① is shown in Figure 16, in which a stainless tube heated by silicon rubber heater to simulate the solar radiation is provided in the basic Rankine cycle. The heat transfer coefficient in the heating test section at different operation condition of the system was investigated in the study. The local heat transfer coefficient of supercritical CO2 (hx) in heating tube (test section) can be evaluated by measuring temperature at the inner wall of tube surface (TIW), temperature at inlet of heating tube (TIN) and input heat flux of heating tube (Qr). hx is thus expressed by

(8)

where TIW can be defined as

(9)

and where TOW is outer wall temperature of the test section (positioned ③ in Figure 16), λSUS is the thermal conductivity of the stainless heating tube, and d1 and d2 are inner and outer diameter of heating tube, respectively.

In Figure 17, representative variation of local heat transfer coefficient of the liquid to the supercritical state in the tube is plotted. It is seen that the bulk temperature (Tb) is increased along the tube, and reaches the critical temperature through the entrance region, in which it keeps stable in the middle of tube. Then Tb increases above the critical temperature (Tpc) at the outlet region. Heat transfer coefficients of CO2 (hx) quickly decreases at the inlet region along the heating tube, but keeps at high value due to higher Prandtl number (Pr) (high heat transfer mode) and reaches the first peak at the peak point of Prandtl number, x/L = 0.3. The second peak is found at x/L = 0.7, and it might be caused by the heat transfer with hydrodynamic interaction phenomena (Shiralkar & Griffith, 1969)

Figure 17. Variation of local heat transfer coefficient hx, of the CO2 along the heated tube at Qr = 3.6 × 103 W/m2 and ṁ = 25 kg/m2s

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