Dwane Paulsen’s Blog

A few years ago I wrote a paper describing a system to measure the volatility of nano-sized aerosol particles (nanoparticles). The particles were exposed to elevated temperatures from approximately 100 to 200°C in a coiled-tube heater. The design was simply a a coil of stainless steel tubing housed in an aluminum cylinder. In the wall of the cylinder were four (4) cartridge heaters. A schematic of the heater is shown in Figure 1.

Figure 1: Volatility-TDMA coiled-tube heater

An interesting property of coiled-tube flow is the enhancement of the heat and mass transfer by a secondary flow formation. This property has been thoroughly explored and heat transfer correlations for coiled-tube flow are available to assist in system design [Futagami and Aoyama, 1988; Mori and Nakayama, 1965; 1967; Patankar et al., 1974]. Figure 2 shows the secondary flow pattern which occurs in the cross-sectional plane normal to the main flow. Compared to straight-tube flow, coiled tubes yield higher heat transfer coefficients and induce radial mixing. For instance, conclusions from numerical and experimental results show that the thermal entrance lengths in coiled tubes are expected to be 20–50% smaller than those of straight tubes [Kakaç, 1987].

Figure 2: Secondary flow pattern common of coiled-tube flow

I performed thermal finite element analysis (FEA) and computational fluid dynamics (CFD) of the heater and coiled tube.  The steady-state FEA model of the heater shows the temperature distribution for a design setpoint of 100°C. The air flow rate through the coiled tube is 1 l/min.

Futagami, K. and Y. Aoyama (1988). Laminar heat-transfer in a helically coiled tube. International Journal of Heat and Mass Transfer, 31(2):387–396. 

Kakaç, S. (1987). Handbook of single-phase convective heat transfer. Wiley, New York. 

Mori, Y. and W. Nakayama (1965). Study on forced convective heat transfer in curvedpipes (1st report, laminar region). International Journal of Heat and Mass Transfer, 8:67–82. 

Mori, Y. and W. Nakayama (1967). Study on forced convective heat transfer in curved pipes (3rd report, Theoretical analysis under condition of uniform wall temperature and practical formulae). International Journal of Heat and Mass Transfer, 10(5):681–695. 

Patankar, S. V., V. S. Pratap, and D. B. Spalding (1974). Prediction of laminar-flow and heat-transfer in helically coiled pipes. Journal of Fluid Mechanics, 62:539–551.

Paulsen, D., E. Weingartner, M. R. Alfarra, U. Baltensperger (2006). Volatility measurements of photochemically and nebulizer-generated organic aerosol particles. Journal of Aerosol Science, 37:1025-1051.