A numerical study on heat transfer characteristics in a spray column direct contact heat exchanger

Abstract

A reliable computational heat transfer model has been investigated to define the heat transfer characteristics of a spray column direct contact heat exchanger, which is often utilized in the process involving counterflows for heat and mass transfer operations. Most of the previous studies investigated are one-dimensional unsteady solutions based on rather fragmentary experimental data. Development of a multidimensional numerical model and a computational algorithm are essential to analyze the inherent multidimensional characteristics of a direct contact heat exchanger. The present study has been carried out numerically and establishes a solid simulation algorithm for the operation of a direct contact heat exchanger. Operational and system 'parameters such as the speed and direction of working fluid droplets at the injection point, and the effects of aspect ratio and void fraction of continuous fluid are examined thoroughly as well to assess their influence on the performance of a spray column. Nomenclature ----------b : Height of lateral inlet opening (m) CD : Drag force coefficient D : Diameter of spray column (m) d : Diameter of dispersion platetm) F; : Interfacial resistance coefficient (kg/sec) g : Acceleration due to gravity(9.81 m/sec") H : Enthalpy(kJ/kg) K : Thermal conductivity(kJ/m . sec' 'C) L : Length of spray column (m) I : Length of inlet piping (m) Nd : Number of droplet Nu : Nusselt number P : Pressurefbl/rrr') Pr : Prandtl number R : Droplet radius (m) Re : Reynolds number T : Temperature CC) Uv : Volumetric heat transfer coefficient (W/ m 3 • 'C) V : Velocityfm/sec) a : Thermal diffusivitytm'i/sec) e : Volume fractions of continuous fluid f1. : Viscosityfkg/m • sec) p : Densitytkg/m") ¢ : Volume fraction of dispersed fluid Subscript c : Continuous fluid d : Dispersed fluid r : Radial coordinate x : Axial coordinate A Numerical Study on Heat Transfer Characteristics in a Spray Column->-345 nt_a between dispefsed floidard ;;I /;~COfthJous floid o 0 o o o 0 0 0 0 o o 0 o 00 0 0 0 0 o o ConlhJous! tOspersed floid outlet IkJidIrlet Fig. 1 Spray column direct contact heat exchanger

Analytical Modelling of a Spray Column Three-Phase Direct Contact Heat Exchanger

“…An analytical model has been developed for the temperature distribution of a spray-column, three-phase direct contact heat exchanger. According to the model developed in this work, we have shown that it is reasonable to assume a constant holdup ratio along the direct contact column, which is in accordance with other numerical models in the literature such as Cabon and Boehm [8], Jacobs and Golafshani [9], and Kang et al [11]. Even more, it has been shown that the vaporization ratio of drops is an influential parameter in the heat exchange process.…”

“…where denoted the holdup ratio or the ratio between the volumes of dispersed phase in the column and the total volume (dispersed and continuous phase volume) and is the column diameter. Substituting (11) into (8) and 9results in = − ( 6 ) . ,…”

“…Their results indicated that the volumetric heat transfer coefficient increases with increasing initial temperature and superficial velocity of continuous phase, while the superficial dispersed phase velocity had no effect. More accurate numerical model is carried out by Kang et al [11] to study the heat transfer characteristics of a spray column direct contact heat exchanger. A twodimensional axisymmetric two-component flow model was developed and they found the injection velocity of dispersed phase has a dominated effect more than other parameters and the volumetric heat transfer coefficient is uniform until the middle column is reached.…”

The Effect of Heat Exchange Fluid Composition on the Performance of a Liquid Nitrogen Engine System

“…Liquid-gas direct-contact heat and mass transfer processes can be found in numerous industrial applications such as water desalination by freezing, geothermal power energy production, crystallisation, waste heat recovery, energy storage systems, solar power energy and emergency cooling of chemical and nuclear reactors [14][15][16][17][18]. Due to vastly different geometries and potentially highly turbulent flow fields (as in the LN 2 engine), numerical models of direct contact heat exchangers in the literature [19][20][21][22] were applicable only to simplified conditions, and very limited quantitative experimental results were available for their validation [20]. In addition, most numerical models [19][20][21][22] have assumed constant fluid properties throughout the flow field, which could lead to large errors, e.g., in cases where the fluid temperature and compositions change significantly [19].…”

“…Due to vastly different geometries and potentially highly turbulent flow fields (as in the LN 2 engine), numerical models of direct contact heat exchangers in the literature [19][20][21][22] were applicable only to simplified conditions, and very limited quantitative experimental results were available for their validation [20]. In addition, most numerical models [19][20][21][22] have assumed constant fluid properties throughout the flow field, which could lead to large errors, e.g., in cases where the fluid temperature and compositions change significantly [19].…”