In the present study, CO 2 gas and tetra-n-butyl ammonium bromide (TBAB) is adopted to form CO 2 +TBAB double hydrate at mass fractions w = 0.05 and 0.10. The moles of CO 2 +TBAB double hydrate are calculated by the variation of pressure of CO 2 gas from initial pressure at about 4.03 MPa and the solubility of CO 2 in TBAB aqueous solution at various subcoolings of 2.0−5.0 K. In addition, the formation characteristics of CO 2 +TBAB double hydrate are observed by the visualization, and it is verified to be a type B double hydrate. The kinetics of CO 2 +TBAB double hydrate formation at w = 0.05 and 0.10 is determined by the normalized rate of hydrate formation in the hydration process.

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d Inner diameter of the test tube (m) d h Hydraulic diameter Fo = aτ/d 2 , Fourier number h Heat transfer coefficient (W/m 2 K) I Current flowing through a standard resistance (A) L Heated length (m) L e Entrance length (m) Nu d Nusselt number P Pressure (kPa) P in Pressure at inlet of heated section (kPa) P ipt Pressure measured by inlet pressure transducer (kPa) P out Pressure at outlet of heated section (kPa) P opt Pressure measured by inlet pressure transducer (kPa) Pr = c p μ/λ, Prandtl numbeṙ Q Heat input per unit volume (W/m 3 ) Q 0 Initial exponential heat input (W/m 3 ) q Heat flux (W/m 3Confidence level (-) ΔT L = (T s − T L ), temperature difference between heater inner surface temperature and average bulk liquid temperature (K)Abstract Steady and transient heat transfer coefficients for water flowing in small tubes with exponentially increasing heat inputs were measured. Platinum tubes with inner diameters of 1.0 and 2.0 mm were used as test tubes, which were mounted vertically in the experimental water loop. In the experiment, the upward flow velocity ranged from 2 to 16 m/s, and the corresponding Reynolds numbers ranged from 4.77 × 10 3 to 9.16 × 10 4 at the inlet liquid temperatures ranged from 298 to 343 K. The heat generation rate exponentially increased with the function. The period of the heat generation rate ranged from 24 ms to 17.5 s.Experimental results indicate that steady heat transfer coefficients decreased with the increase in the inner diameter of the small tube. Moreover, the ratio of bulk viscosity to near-wall viscosity of water increased with the rise in surface temperature of the vertical tube. From the experimental data, correlations of steady-state heat transfer for inner diameters of 1.0 and 2.0 mm were obtained. The heat transfer coefficient increased with decreasing the period of the heat generation rate as the flow velocity decreased. Moreover, the Nusselt number under the transient condition was affected by the Fourier number and the Reynolds number. List of symbolsA Surface area (m 2 ) a Thermal diffusivity (m 2 /s) BBasis limit (-) c Specific heat (J/kg K) c p

The turbulent heat transfer, the subcooled boiling heat transfer and the steady state CHF for a Pt-circular test tube of a 3 mm inner diameter and a 100 mm heated length are measured with a wide range of inlet subcooling and flow velocity at high liquid Reynolds number, i.e. Re d =3.0110 4 to 1.4310 5. The inner surface temperature of the Pt-circular test tube calculated by the steady one-dimensional heat conduction equation is compared with  Corresponding author. the values derived from authors' turbulent heat transfer correlation and with the numerical solutions of the RANS equations (Reynolds Averaged Navier-Stokes Simulation) of k- turbulence model for the flow velocities ranging from 4 to 21 m/s. The thicknesses of conductive sub-layer from non-boiling regime to CHF are measured by numerically analyzing the heat transfers with conductive sub-layer on forced convection and with thinner one dissipated by the evaporation on nucleate boiling. The thicknesses of viscous sub-layer on forced convection are estimated from the thicknesses of the conductive sub-layer and Prandtl numbers of the surface temperature on the heated surface. Furthermore, the thicknesses of conductive sub-layer at the CHF point are extrapolated from the measured values at various flow velocities. The experimental values of the CHF are also compared with authors' widely and precisely predictable correlations of critical heat flux during flow boiling of subcooled water and the corresponding theoretical values of the liquid sub-layer dry-out models suggested by other researchers, respectively. The authors' correlations and other researchers' theoretical values can represent the subcooled boiling CHFs obtained in this study within the ranges of-13.27 to 6.76% difference and-32.51 to 13.16 % one, respectively. A suggestion based on the experimental data as to what the dominant mechanism is for critical heat flux during flow boiling of subcooled water on a vertical circular tube is confirmed again at high liquid Reynolds number. The transitions to film boiling at the subcooled water flow boiling on the Pt test tube of d=3 mm and L=100 mm would occur due to the liquid sub-layer dry-out model at the steady-state CHF as well as those on the Pt test tube of d=3 mm and L=66.5 mm, but not due to the heterogeneous spontaneous nucleation and the hydro-dynamic instability.