Jiafeng Wu

Three-Dimensional Numerical Simulation for Annular Condensation in Rectangular Microchannels

A three-dimensional model in rectangular microchannels with constant heat flux is developed to predict steady annular condensation. The condensate flow field on the side wall, which is dominated by surface tension, is divided into two regions: the thin–film region and the meniscus region. The momentum and mass equations, in both the vapor and meniscus regions, along with the film thickness equation in thin–film region are solved numerically. The distribution of the meniscus curvature radius, thickness of the condensate film, heat transfer coefficient, and wall temperature are all determined. The results indicate that with the development of condensation, the condensate in the thin–film assumes a convex profile shape at the side wall, with the crest located at the midpoint of the side wall. The film thickness in the thin-film region increases at upstream locations and decreases as the flow moves downstream. The average heat transfer coefficient in the thin-film region is much larger than that occurring in the meniscus region. And the highest local heat transfer coefficient occurs at the intersection of the thin-film region and the meniscus on a cross section where the maximum wall temperature exists. The circumferential average heat transfer coefficient decreases drastically upstream to a lower value. After that, it remains nearly constant until close to the end of the annular flow, where it again begins to decrease.

Experimental Study on Shell Side Heat Transfer Performance of Circumferential Overlap Trisection Helical Baffle Heat Exchangers

A set of experiments were conducted on the circumferential overlap trisection helical baffle heat exchangers with inclined angles of 20°, 24°, 28° and 32° single-thread and inclined angle of 32° dual-thread one, and a segmental baffle heat exchanger as a contrast scheme. The cylinder case of the testing heat exchanger is a common shell, while the tube bundle core could be replaced. The shell side heat transfer coefficient ho is obtained by subtract tube-side convection thermal resistance and tube wall conduction resistance from the overall heat transfer coefficient K. The curves of shell side heat transfer coefficient ho , pressure drop Δpo , Nusselt number Nuo , and axial Euler number Euz,o are presented versus axial Reynolds number Rez,o . A comprehensive performance index Nuo /Euz,o is suggested to demonstrate the integral properties of both heat transfer and flow resistance of different schemes, and the curves of Nuo /Euz,o versus Rez,o of the different schemes are presented. The results show that the scheme with inclined angle 20° performs better than other schemes, and the scheme with inclined angle 24° ranks the second, however the segment scheme ranks the last. The curves of Nuo /Euz,o of both schemes with inclined angle 32° of single-thread and dual-thread are almost coincident, even though their heat transfer coefficient and pressure drop curves are quite different. The results indicate also that for the circumferential overlap trisection helical baffle schemes the optimal inclined angle is around 20° instead of around 40° as rated by many literatures for the quadrant helical baffle schemes.Copyright

Influence of baffle configurations on flow and heat transfer characteristics of unilateral type helical baffle heat exchangers

Abstract The flow and heat transfer performances were investigated on unilateral ladder type helical baffle heat exchangers (ULHBHXs) with different geometrical structures in half cylindrical space and compared with those of the segmental baffle heat exchangers (SBHXs). The unilateral ladder type helical baffles consist of two alternate groups of plates, the folded baffles (Baffles A) and the segment ones (Baffles B). The studied ULHBHXs include the combination schemes of different Baffle A with different baffle pitches (40, 60, 80u202fmm) and different projection length of inclined section (lu202f=u202f60, 72, 84u202fmm and a plane one). Visualization methods of nephograms were adopted on special slices to clearly reveal the impacts of baffle configurations on flow patterns. The numerical results show that the shell-side heat transfer coefficients of the ULHBHX schemes with folded Baffles A are superior but the comprehensive indexes are inferior to those of the corresponding plane ones, and all the helical schemes are superior to those of the segmental schemes. The average ratios of shell-side heat transfer coefficient, pressure drop and comprehensive indexes hoΔpo−1 and hoΔpo−1/3 of all the twelve ULHBHX schemes over those of the SBHX ones with corresponding baffle pitches are 1.166, 0.670, 1.851 and 1.352, respectively.

Shell Side Flow and Heat Transfer Performances of Trisection Helical Baffle Heat Exchangers

The flow and heat transfer performances of three trisection helical baffle heat exchangers with different baffle shapes and assembly configurations, and a continuous helical baffle scheme with approximate spiral pitch were numerically simulated. The four schemes are two trisection helical baffle schemes of baffle incline angle of 20° with a circumferential overlap baffle scheme (20°TCO) and a end-to-end helical baffle scheme (20°TEE), a trisection mid-overlap helical baffle scheme with baffle incline angle of 36.2° (36.2°TMO), and a continuous helical baffle scheme with baffle helix angle of 16.8° (18.4°CH). The pressure or velocity nephograms with superimposed velocity vectors for meridian slice M1, transverse slices f and f1, and unfolded concentric hexagonal slices H2 and H3 are presented. The Dean vortex secondary flow field, which is one of the key mechanisms of enhancing heat transfer in heat exchangers, is clearly depicted showing a single vortex is formed in each baffle pitch cycle. The leakage patterns are demonstrated clearly on the unfolded concentric hexagonal slices. The results show that the 20°TCO and 18.4°CH schemes rank the first and second in shell-side heat transfer coefficient and comprehensive indexes ho/Δpo and ho/Δpo1/3. The 20°TEE scheme without circumferential overlap is considerably inferior to the 20°TCO scheme. The 36.2°TMO scheme is the worst in both shell-side heat transfer coefficient and comprehensive index ho/Δpo1/3.Copyright