An Experimental Study on Heat Transfer Coefficients of a CO2-Filled Thermosyphon

Abstract

Because carbon dioxide is ozone friendly and has negligible global warming potential, it has received renewed interest in recent years as an important alternative refrigerant. In this article, the heat transfer characteristics of a carbon dioxide-filled two-phase closed thermosyphon were investigated experimentally, and the empirical heat transfer correlations are reported. The heat transfer data were analyzed, and heat transfer coefficients were compared with conventional heat transfer correlations. The results represent that heat transfer correlation of the heated section can be expressed with Kutatelatze's correlation, and heat transfer coefficients of the condenser section are found to be in reasonable agreement with the Nusselt equation.     

Empirical Study of an Air-to-Air Heat Pipe Heat Exchanger in Tropical Climates

Abstract

This article presents the empirical study of a heat pipe heat exchanger that was directly experiencing the ambient tropical air in its evaporator section. The performance of the heat pipe heat exchanger was monitored during two weeks of operation to determine the performance curves. The temperature of return air was controlled at about 22°C as the representative temperature of inside air, and a face velocity of 2 m/s on the heat pipe heat exchanger coil was established for this purpose. It was found that for the present investigation, the heat pipe heat exchanger can pay for itself in 1.5 years.     

Application of a Factorial Design Method to Determine the Accuracy of a Solution to an Inverse Heat Conduction Problem

A numerical/experimental inverse procedure was employed to estimate the temperature-dependent thermal conductivity of a solid body in which 1D heat conduction in a top heated cylindrical sample is assumed. The emphasis is focused on the issue of sensitivity of results to selected assumptions made in inverse calculations. It has been found that the accuracy of heat capacity evaluation brings the largest contribution to final errors (up to 74%). Density accounts for one-fourth to one-third of the total error of determination. The failure to ensure unidirectional heat conduction in a sample during an experiment is important only at elevated temperatures.     

Experimental Investigation of Saturated Flow Boiling Heat Transfer to TiO2/R141b Nanorefrigerant

Heat transfer in two-phase flow boiling of a dilute mixture of TiO₂ nanoparticles in R141b base fluid in a smooth tube is investigated experimentally. Examining the obtained results reveals that enhancement of the convective heat transfer coefficient for the particle volume fractions of 0.01% and 0.03% in comparison with pure R141b is more pronounced for a higher volume fraction. The measured data also show that at higher vapor qualities, the improvement in heat transfer coefficient is greater. Moreover, the heat transfer coefficient decreases substantially with mass flux while an increase in saturation temperature leads to an improvement in this coefficient.     

Enhancement in nucleate pool boiling heat transfer on nano-second laser processed copper surfaces

ABSTRACT

The present paper reports the experimental investigation of pool boiling heat transfer on multiscale functionalized copper surfaces. Multiscale functionalized surfaces are fabricated by employing the nano-second laser surface process (NLSP) technique. The heat transfer coefficients (HTCs) of functionalized surfaces are estimated experimentally by using water and acetone as pool liquid. Tests are performed at atmospheric pressure, and saturated pool boiling condition with heat flux varyies between 0 and 330 kW/m². The maximum HTCs for functionalized surface and reference polished surface were found to be 41,500 W/m²K and 23,000 W/m²K, respectively, with water and 22,000 W/m²K and 14,000 W/m²K, respectively, with acetone.     

BOILING IN CHANNEL WITH POROUS MATRIX

Experimental data on heat transfer and pressure drop in the case of forced-convection boiling in a channel with a porous matrix are presented. Correlations for the experimental data are given. Example calculation of thermohydrautic characteristics of a heat exchanger with a highly conductive porous matrix has been done using the obtained experimental data. Results of these calculations are presented.     

Polymerization-induced Phase Separation: Phase Behavior Developments and Hydrodynamic Interaction

The process of polymer synthesis based on polymerization-induced phase separation (PIPS) is revisited from the theoretical point of view. Cahn–Hilliard–Cook theories for spinodal decomposition are adapted to describe the kinetics of phase separation and deduce the time-resolved scattering function, while the double reaction model is used to describe the kinetics of polymerization. Coupling of these two kinetics is provided by the Carother's equation relating the fraction of reacted monomers to the degree of polymerization at time t, denoted N(t). It is argued that the approach to criticality is governed by a critical parameter, χ_c, that is different from the usual parameter for spinodal decomposition, χ_s, deduced from the second derivative of the free energy. While the latter parameter depends on the reciprocal degree of polymerization N^?1(t), the former one depends on its time integral. This leads to significant consequences on the phase behavior developments during the PIPS process. Hydrodynamic interactions are found to speed up the emergence of instability modes. Although the qualitative trends remain similar to those of the Rouse dynamics, important quantitative changes are found due to the long-range viscous flow effects.     

The critical behavior of Kac-type models for semi-infinite systems

The critical behavior of the layer magnetizations and local susceptibilities of theD-vector lattice models with Kac-type ferromagnetic interactions for a semi-infinite system is studied. These local quantities behave less singularly than the bulk ones, showing that this is not peculiar to the two-dimensional Ising model. Moreover, the limiting form (at the critical point) of the magnetization profile can be obtained, which, when properly scaled, satisfies the minimum condition in the Landau theory for a semi-infinite continuous system. Landau-type critical behavior is thus recovered.     

The space localization of unbounded boundary perturbations in nonlinear heat conduction with transfer

We consider the nonlinear parabolic equation u_t = (k(u)u_x)_x + b(u)_x, where u = u(x, t, x ε R¹, t > 0; k(u) ≥ 0, b(u) ≥ 0 are continuous functions as u ≥ 0, b (0) = 0; k, b > 0 as u > 0. At t = 0 nonnegative, continuous and bounded initial value is prescribed. The boundary condition u(0, t) = Ψ(t) is supposed to be unbounded as t → +∞. In this paper, sufficient conditions for space localization of unbounded boundary perturbations are found. For instance, we show that nonlinear equation u_t = (uⁿu_x)_x + (u^β)_x, n ≥ 0, β >; n + 1, exhibits the phenomenon of “inner boundedness,” for arbitrary unbounded boundary perturbations.