Research PaperThe effect of Dixon rings on direct contact heat transfer performance: Comparison of counter and co-current evaporation
Graphical abstract
Introduction
It is an undisputed fact that the energy crisis has become one of the greatest concerns today. There are two solutions to solve the problem mentioned above. The first method is to develop new energy technologies, such as solar energy [1] and wind energy [2]. Another effective approach is utilizing energy scientifically and supplying energy efficiently based on the idea of sustainability.
As an effective way to use energy, direct-contact heat exchanger was widely studied after its impressive investigation by Sideman et al. [3], [4], [5], [6], [7], [8] Direct contact heat transfer has many superiorities such as simple construction, fairly high heat transfer coefficient, negligible fouling and corrosion [9], [10], [11]. On account of those merits, it has been applied and researched extensively in crystallization [12], desalination [13], energy recovery from industrial waste [14], [15], ice-slurry production [16] and thermal energy storage [17].
As one of the most important applications in the field of direct contact heat transfer, direct contact evaporation also has been the object of study in recent decades. A lot of scholars studied the evaporation process of a single droplet on the micro-level with the aid of high-speed digital camera and cine-photographic technique [13], [18], [19], [20], [21]. As for swarms of droplets, Smith et al. [11] developed an analytical model for calculating volumetric heat-transfer coefficients for direct-contact evaporation by using single droplet correlations. Song and Steiff [22] presented a model to simulate vaporization height based on the concept of phase space and the population balance equation. Siqueiros and Bonilla [23] investigated the factors that were closely related to volumetric heat transfer coefficient. Song et al. [24] presented a population balance model to predict the volumetric heat transfer coefficient for direct-contact evaporation in a bubble column based mainly on the energy balance and the population balance. Fu et al. [25] investigated the behaviors of drobbles in the experiment of the pentane evaporating in hot water. In general, evaporation in bubble column was well studied in the past few decades.
The evaporation in spray-column was first studied by Sideman and Gat [3]. Battya et al. [26] did a simple theoretical analysis on the contact latent heat transfer in a counter flow spray column. However, there is little study on it owing to flooding and strict operational condition [27]. Seetharamu and Battya [28] investigated the evaporation of R-113 in a stagnant column of distilled water, and developed correlations to predicate heat transfer coefficient. But the limiting condition for keeping hot water stagnant made it unacceptable in practical application. Mori [29] proposed a model to calculate heat transfer coefficient assuming no nucleation. Hameed [30] gave a systematic analysis of evaporation in a spray column, but the assumption that the holdup ratio was constant along the column was far-fetched.
After careful mathematical analysis, Sideman and Moalem [31] pointed out that counter-current was more efficient than co-current operation in direct contact condensation in the column. Park et al. [32] investigated the direct contact condensation in horizontal circular pipe, and found that the overall heat transfer characteristics was better in the concurrent flow.
As an effective device, the packing column was widely studied in the process of direct contact condensation, and played a positive role in the process of heat transfer [33], [34], [35], [36]. More recently, in our lab, Jiang et al. [37] investigated the effect of Dixon rings on the performance of direct contact evaporation in bubble column (co-current), and found that packing played a good role in promoting heat transfer. But the experiments just involved the effects of packing on the bubble column. The investments on counter-current flow were not covered. And it is worthy of lucubrating.
In the direct contact heat transfer of volatile liquid evaporation in an immiscible liquid, spray column and bubble column are nearly identical in the operation except flow direction of the continuous phase. In the former, it is co-current flow, while counter-current flow in the latter. So the comparison between those two different operations can provide guidance for choosing suitable contact form in industrial application. Unfortunately, there are little researches on it. At the same time, enhancing heat transfer by utilizing packing in two operations listed above is also worth of investment. And evaluating the effect contributes to our understanding of the influence of packing. In this work, we devote to investigating the heat transfer performance in both co-current and counter-current operations under the condition of adding packing or not, and then making comprehensive and comparative evaluations and analyses.
Section snippets
Experimental setup
Taking safe and economic aspects into account, industrial pentane (molar ratio of n-pentane and iso-pentane is 1:1, the physical properties are shown in Table 1) and tape water were chosen as working fluids. Pentane was used as volatile dispersed phase, and water immiscible continuous phase. Before pentane was injected into water by a diaphragm pump, it was preheated to a temperature near the boiling point.
The schematic diagram of the experimental setup is illustrated in Fig. 1. The
Heat transfer quantity
In the experiment, heat loss couldn’t be avoided completely even some necessary insulation measures had been taken. When the hot water flowed through the column body, the temperature drop was no more than 0.5 °C. Compared with the quantity of heat used to evaporate pentane, so much heat loss couldn’t be ignored. At the same time, if the temperature variation is not significant, sensible heat is negligible when compared to latent heat. And because the pentane was preheated to the temperature near
The axial temperature distribution
The axial temperature of the continuous phase is the authoritatively representing index of heat transfer performance. The rate of temperature change signifies the heat-transfer process. After the liquid pentane outflowed from the distributor, it contacted with high-temperature water and evaporated immediately. When it was co-current flow, water and pentane flowed upward simultaneously. The pentane absorbed heat continuously from the water, and evaporated gradually until all liquid pentane had
Conclusion
In this paper, a series of experiments were performed to investigate some factors that had influences on direct contact evaporation. Those factors include flow rate of the continuous phase and the dispersed phase, temperature difference, packing and flow direction. And according to the experimental results, we put forward some guiding suggestions.
In the experiments of direct contact evaporation, counter-current flow was always superior to co-current flow in both optimal column height and
Acknowledgements
The authors gratefully acknowledge Dr. Xue Kang in Green Energy Laboratory for her contribution to language editing and results analysis to this paper.
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These authors contributed equally to this work.