Thermal Performance Enhancement of Triplex Tube Latent Thermal Storage Using Fins-Nano-Phase Change Material Technique

Latent heat thermal energy storage (LHTES) systems using a phase change material (PCM) can reduce the heat-transfer rates during charging/discharging processes because of their inherently low thermal conductivity. In this study, heat-transfer enhancement using various configurations of longitudinal fins employing both a PCM and a nano-PCM in a large triplex-tube heat exchanger (TTHX) was numerically investigated via the Fluent 15 software. The results showed that the thermal conductivity of the pure PCM (0.2 W/m K) can be observably enhanced by dispersing 10% alumina (Al₂O₃) to 25%. Therefore, the melting time is reduced to 12%, 11%, and 17% for the internal, internal-external, and external fins, respectively, compared with the case of the PCM without nanoparticle. It is concluded that the model of external fins-nano-PCM embedded in a large TTHX is the most efficient model for achieving complete PCM melting in a short time (188 min), where improving the thermal performance to 14% and 11% compared with the TTHX with internal and internal-external fins-nano-PCM, respectively. The simulation results are validated and agree well with experimental results for the PCM and nano-PCM.     

Heat Transfer Enhancement of Spray Cooling on Flat Aluminum Tube Heat Exchanger

This study provides an experimental analysis on the heat transfer performance of a flat aluminum tube microchannel heat exchanger with spray cooling. The effects of water spraying rate, airflow rate, and relative humidity were investigated. The test results show that the heat transfer performance increased with increasing the water spraying rate but without the penalty of increased flow resistance at low spray conditions. This effect is further enhanced by increasing the water spraying rate. However, when the spraying rate is high, part of the nonevaporated drops attached to the fin surface and formed a liquid film, which caused the flow passage to become narrower. Further increase in the spraying rate resulted in part of the flow passages being blocked by the nonevaporated water drops and caused a region of poor heat transfer. The friction coefficient jumped drastically at this condition. This phenomenon deceased gradually with increasing airflow rate. High inlet air humidity resulted in the water accumulation phenomenon appearing at lower water spraying rates. The evaporative cooling effect decreased and flow friction increased. The test results just described show that the water spray is able to significantly improve the air-side heat transfer performance. The optimum spray rate for each airflow rate must be carefully determined.     

Heat Transfer Enhancement for Heat Exchanger Network Retrofit

Heat exchanger networks (HEN) play important roles in a chemical plant. In a plant lifetime, it may be required to retrofit a HEN several times in order to improve the energy efficiency or to accommodate the increase in throughput. The network pinch method developed by Asante and Zhu [1] can identify bottlenecks, which limit the increase in heat recovery for an existing HEN and also indicate promising structure changes to overcome the bottlenecks. As a result of HEN retrofit, additional surface area is required for some heat exchangers. There are a number of options to provide additional area, such as installing new shells or new units, adding new tubes to an existing bundle, etc. If heat transfer enhancement (HTE) is applied, additional area can be reduced significantly. This can result in a great reduction in capital cost and implementation time for modifications. However, in practice, heat transfer enhancement techniques have not been applied extensively, particularly in the petroleum refining industry. Several main aspects need to be addressed when HTE is taken into consideration for HEN retrofit. The first is how to determine which heat exchangers are suitable to apply HTE in the network and the second issue is to determine what level of augmentation of heat transfer performances is required. The last is about how to select a particular enhancement technique that can fulfil the enhancement requirement. A new strategy for applying HTE in HEN retrofit at the conceptual design stage has been developed. The above issues can be addressed properly by this new method. The new procedure is demonstrated using a case study.     

三叶膨胀管换热器壳程强化传热的数值研究

三叶膨胀管是一种新型强化传热管,针对纵向流换热器特点,设计了三种不同管束结构参数的三叶膨胀管自支撑纵向流换热器。应用FLUENT软件及Realizable k-ε湍流模型,对三种不同结构参数的三叶膨胀管换热器壳程强化传热特性展开了数值模拟,并通过与实验数据的对比,验证了计算模型的可靠性。计算了不同壳程介质流速下,三叶膨胀管换热器壳程的换热系数与压降值,并获得了壳程流体流线以及相应的温度场、速度场和二次流分布图。结果发现,在壳程水流速一致的情况下,管束横向间距越大的三叶膨胀管换热器,壳程拥有更高的综合换热性能和更低的压降值,但相应地,换热系数也更低。流场分析显示,壳程流体流线呈现出三维纵向旋流形态,二次流的出现改变了速度场和温度场分布,二次流的强度随着管束横向间距的减小而增大。     

扭曲椭圆管换热器壳程强化传热的数值研究

基于扭曲椭圆管的换热器是一种新型的新风系统换热器,针对扭曲椭圆管及其应用特点,设计了两种不同结构参数的新风系统换热器。应用FLUENT软件,在夏季工况下对两种不同结构参数的新风系统换热器壳程进行模拟分析,并通过与实验数据的对比,验证计算模型的可靠性。结果显示在相同体积流量下,随着壳程开孔面积的增大,对流换热系数h不断减小,压降Δp不断减小,综合性能系数h/Δp1/3变化不明显;随着螺距的减小,对流换热系数h不断增大,压降Δp不断增大,综合性能系数h/Δp1/3也不断增大;流场分析显示,扭曲椭圆管换热器壳程流道内,呈现出明显沿着扭曲椭圆管壁面的螺旋流,使得空气在流道内充分扰动,增强换热效果。     

管壳式换热器的换热管强化传热技术浅述

介绍了管壳式换热器的换热管强化传热技术,分析了各自的原理、优缺点及推荐的使用场合。采用节能技术的换热器不仅提高了能源的利用率,而且减少了金属材料的消耗,对化工行业提高经济效益具有重要意义。     

自激振荡管壳式换热器强化传热的可行性分析

脉动流体能够使管壳式换热器换热系数得到提高,而自激振荡腔在一定的结构参数和运行参数下能够使流体产生脉动。在管壳式换热器前安装自激振荡腔,使流体流经自激振荡腔产生脉动流动,从而实现管壳式换热器的强化传热。分析了将自激振荡腔用于管壳式换热器强化传热可行性。     

采用肋片强化相变蓄热器传热的模拟分析

为了增加同心套管式相变蓄热器的蓄能效果,采用环形肋片强化相变储能设备的传热,利用Fluent软件模拟了这种结构中石蜡相变的融化过程,得到了石蜡熔化过程温度场分布及熔化时间的规律,根据这些规律分析了肋片间距及厚度等参数对贮热管放热效果的影响。分析结果表明：石蜡的总融化时间随肋片间距增加而延长即传热效果变差,但是随着肋片厚度的增加而缩短,即传热效果变优,但是当或肋间距超过40mm和厚度超过2mm后,进一步增加肋片间距或者厚度对传热效果的影响变得不明显。     

双管程热交换器换热特性研究

通过对双管程热交换器结构和工作原理介绍,利用经典对流换热理论,采用定性判断和定量比较的方法,分析双管程热交换器与管壳式换热器的换热能力。对双管程热交换器而言,换热能力上,竞争优势明显;流动阻力上,无明显优势。在低品位能源余热利用方面,对深化双管程热交换器的研究,优化设备结构、降低成本有一定意义,值得工程技术人员借鉴与参考。     

High Performance Tubular Heat Exchanger with Minijet Heat Transfer Enhancement

ABSTRACT

In the paper, the original cylindrical heat exchanger with minijets (MJHE) was introduced. The systematic experimental analysis of the prototype heat exchanger was described with special attention paid to such parameters as the heat transfer effectiveness, heat transfer rates, overall heat transfer coefficients, and pressure drop. The heat transfer coefficients were determined based on Wilson plot method, the most suitable approach for heat transfer coefficient determination in exchangers of complex geometry. The thermal-hydraulic characteristics of the in-house manufactured prototype of MJHE in water–water and gas–water configuration are also presented. The experimental results were compared with the predictions from well-known correlations found in the published research papers dedicated to the free-surface and submerged types of jets.     

Enhancing Heat Transfer and Energy Storage Performance of Shell-and-Tube Latent Heat Thermal Energy Storage Unit with Unequal-Length Fins

Previous studies in literatures adequately emphasized that inserting fins into phase change material is among the most promising techniques to augment thermal performance of shell-and-tube latent heat thermal energy storage unit.In this study,the novel unequal-length fins are designed from the perspective of synergistic benefits of heat transfer and energy storage performance,and the effects of arrangement,number and total length of unequal-length fins are numerically investigated.Results show t...     

Heat Transfer during Annular Tube Contact in a Helically Coiled Tube-in-Tube Heat Exchanger

Abstract

Helically wound tube-in-tube heat exchangers are manufactured by coiling two tubes, one placed inside the other. This method often results in the tubes not sharing the same center line, and therefore annular contact occurs in some cases. An experimental comparison was made of such tubes in a heat exchanger with annular contact, as opposed to an aligned (concentric) device without annular contact, in order to quantify the effect of annular contact in terms of heat transfer coefficients and pressure drop. By comparing the heat transfer characteristics, it was concluded that the heat transfer coefficient in the annulus was found to increase substantially. The result was an improved performance by the heat exchanger where annular contact occurs, compared to the heat exchanger with the inner tube in a concentric position.     

Heat Transfer Analysis for a Concentric Tube Heat Exchanger Including the Wall Axial Conduction

The effect of wall axial conduction on the heat transfer in a concentric tube heat exchanger is examined for the inner flow laminar flow regime. The procedure used for the current analysis combines the analytical solution for the inner fluid with a numerical approximation for the wall conduction and has the capability of handling the temperature variation for the outer fluid. Both parallel and counterflow cases are evaluated for the analysis, and results are presented in terms of the axial variations of fluids and wall temperatures. Effects of the heat capacity rate ratio of the fluids on the temperature variations and on the mean heat flux are also pointed out. The effect of the exchanger length is included for the analysis. It is concluded that the total heat transfer between the fluids is greatly influenced by the wall axial conduction for the counterflow arrangement and is not ignorable when the heat capacity rate ratio of fluids are smaller than unity.     

波纹管降膜蒸发器强化传热技术

本文描述了管式换热器的强化传热技术，并从传热机理出发，对波纹管降膜蒸发器的结构特点和强化传热特性进行了分析，阐述了波纹管降膜蒸发器的防除垢性能及其存在的问题。     

Effect of Twin Delta-Winged Twisted-Tape on Thermal Performance of Heat Exchanger Tube

This work aims at studying the effect of twin delta-winged twisted-tape insertion on heat transfer, pressure drop, and thermal performance characteristics of a heat exchanger tube. All twisted tapes used in this work were made of aluminum sheets twisted at a single twist ratio of 3.0. The twin delta wings were formed by extrusion of the tape at the center area at every twist length interval. For comparison, three different arrangements of the twin delta wings were: (1) the wing tips pointing upstream of the flow (TTW-up, twin delta-winged twisted tape in counterflow arrangement), (2) the wing tips pointing downstream of the flow (TTW-down, twin delta-winged twisted tape in co-flow arrangement), and (3) the wing tips pointing opposite direction (TTW-o, opposite winged twisted tape). The wing declination was arranged at an angle of 15° with respect to the tape surface. Effects of three different wing-tip angles of 20°, 40°, and 60° for a constant wing base were examined. The experiments were conducted using water as the test fluid in a uniform-heat-flux tube for Reynolds number between 5000 and 15,000. The results demonstrate that the TTW-up consistently provides greater heat transfer rate, friction factor, and thermal performance factor than the TTW-down and the TTW-o, at a similar condition. In addition, the heat transfer rate increases as the wing-tip angle decreases. Over the range investigated, the TTW-up with wing-tip angle of 20° gives the highest thermal performance factor of 1.26 along with a Nusselt number and friction factor of 2.57 and 8.55 times those of the plain tube.     

Investigation of Excessive Material on Insulating Properties Using Different Heat Transfer Fluid for Thermal Energy Storage Development

Though TES （thermal energy storage） is developed hugely in most of the solar power generation plants, it is less growth in implementing a modular type of TES in a solar plant, e.g., solar dish/stifling engine application. The main issue in designing the TES system is its thermal capacity of storage materials, e.g., insulator. This study is focusing on the potential waste material as an insulator for thermal energy storage applications. The insulator usage is to reduce the heat transfer between two mediums and the capability is measured by its resistance to heat flow. It is needed to obtain optimal materials to energy conversion at the same time reduce the waste generation. Therefore, a small-scale experimental testing of natural cooling process of an insulated tank within a confined room without any forced cooling system, e.g., fan. The testing is repeated by changing the insulator using the potential waste material from natural and industrial waste and also by changing the HTF （heat transfer fluid）. The analysis is performed on the relationship between heat loss and the reserved period by the insulator. The results indicate the percentage of period of the insulated tank withstands the heat compared to non-insulated tank, e.g., cotton reserved the period of 14% more than non-insulated tank to withstand the heat transfer of cooking oil to the surrounding. The paper finally justifies the most potential waste material as an insulator in different heat transfer fluids.     

换热管内转子强化换热的数值模拟

本文概述了转子组合式强化传热装置的强化传热和自清洁原理。分别通过建立光管及内置多个转子换热管的三维流动模型,对换热管内流场、温度场、压力场以及换热过程进行了模拟,得到了管内流体的流动特性和传热特性。对比模拟结果表明,内置转子换热管管内的三维流动比较复杂,转子与管壁之间缝隙内的流体有着明显的环绕流动,切向速度和径向速度也增大到一定范围,转子旋转半径内的流体整体呈螺旋流动。总的来说,内置转子的换热管内较光管有较强的湍流度,尤其是近壁区域,因此强化了管内的对流换热,传热系数显著提高,从而验证了转子具有强化传热和自清洁的双重功能。     

管内插物强化换热性能分析及应用

管内插物的种类很多，扭带、螺旋线圈以及绕花丝是三种较常用的管内插物强化换热技术，对它们的强化换热性能以及应用进行分析比较，是非常有必要的。从综合强化性能来看，螺旋线圈内插物比扭带内插物效果好；绕花丝内插物是一种新型综合强化换热技术，它优于前两种内插物的特点是：可使流体在流动方向上做复杂的三维混合流动，并且所产生的阻力降非常小，所以综合强化换热性能最好。图7表1参10     

Numerical Study of Solid State Hydrogen Storage System with Finned Tube Heat Exchanger

Abstract

Absorption of hydrogen gas inside the metal hydride (MH)-based hydrogen storage system generates significant amount of heat. This heat must be removed rapidly to improve the performance of the system which can be accomplished by embedding a heat exchanger inside the MH bed. In this article, a tubular shape MH system, equipped with a heat exchanger consisting of copper tube and pin fin is presented. A detailed 3D mathematical model is developed using COMSOL Multiphysics 4.3b for the numerical study of absorption and desorption processes inside the storage system. Impact of various operating and geometric parameters on the charging time of the storage system has been examined. It is observed that these geometric and operating parameters influence the charging time of the storage system. In the last, the impact of heat exchanger material on the performance of the storage system is explored. It is found that aluminum made heat exchanger is the best for the storage systems. The absorption process is accomplished in 1152?s at the operating parameters of 15?bar, 298 K, and 6.75 lit/min. This numerical work suggests that the efficient design of storage system is very important for rapid absorption and desorption of hydrogen.