Design of a novel, intensified heat exchanger for reduced fouling rates

This paper describes an integrated approach into the design and evaluation of a novel tube bundle heat exchanger that achieves higher heat transfer levels at lower levels of pressure drop, while remaining less susceptible to gas-side fouling. The approach combines laboratory scale experiments with industrial observations and numerical simulations of full-scale heat exchangers to study the thermal, hydraulic and fouling characteristics of tube bundle heat exchangers. Three arrangements are compared and the advantages of the proposed novel arrangement are demonstrated. Enhanced heat transfer rates are combined with reduced pressure drop and gas-side fouling rates through careful design of the shape of the tube cross-section and reduced transverse spacing.     

Thermally dependent viscosity and non-Newtonian flow in a double-pipe helical heat exchanger

A double-pipe helical heat exchanger was numerically studied to determine the effects of thermally dependent viscosity and non-Newtonian flows on heat transfer and pressure drop for laminar flow. Thermally dependent viscosities were found to have very little effect on the Nusselt number correlations for Newtonian fluids; however significant effects on the pressure drop in the heat exchanger were predicted. Changing the flow rate in the annulus can significantly affect the pressure drop in the inner tube, since the average viscosity of the fluid in the inner tube would change due to the change in the average temperature.The effects of non-Newtonian power law fluids on the heat transfer and the pressure drop were determined for laminar flow in the inner tube and in the annulus. The Nusselt number was correlated with the Péclet number for heat transfer in the inner tube. For the annulus, the Nusselt number was found to correlate best with the Péclet number and the curvature ratio. Pressure drop data were compared by using ratios of the pressure drop of the non-Newtonian fluid to a Newtonian fluid at identical mass flow rates and consistency indices.     

Twisted bundle heat exchangers performance evaluation by CFD (CJ12/5054)

Shell and tube heat exchanger with single twisted tube bundle in five different twist angles, are studied using computational fluid dynamics (CFD) and compared to the conventional shell and tube heat exchanger with single segmental baffles. Effect of shell-side nozzles configurations on heat exchanger performance is studied as well. Heat transfer rate and pressure drop are the main issues investigated in the paper. The results show that, for the same shell-side flow rate, the heat transfer coefficient of heat exchanger with twisted tube bundle is lower than that of the heat exchanger with segmental baffles while shell-side pressure drop of the former is even much lower than that of the latter. The comparison of heat transfer rate per unit pressure drop versus shell-side mass flow rate shows that heat exchanger with twisted tube bundle in both cases of perpendicular and tangential shell-side nozzles, has significant performance advantages over the segmental baffled heat exchanger. Optimum bundle twist angles for such exchangers are found to be 65 and 55° for all shell side flow rates.     

Heat transfer and pressure drop performance of twisted oval tube heat exchanger

Twisted oval tube heat exchanger is a type of heat exchanger that aims at improving the heat transfer coefficient of the tube side and also decreasing the pressure drop of the shell side. In the present work, tube side and shell side heat transfer and pressure drop performances of a twisted oval tube heat exchanger has been experimentally studied. The tube side study shows that the tube side heat transfer coefficient and pressure drop in a twisted oval tube are both higher than in a smooth round tube. The shell side study shows that the lower the modified Froude number Fr_M, the higher the shell side heat transfer coefficient and pressure drop. In order to comparatively analyze its shell side performance of the heat exchanger, a rod baffle heat exchanger with similar size of the twisted oval tube heat exchanger is designed and its performance is calculated with Gentry's method. The comparative study shows that the heat transfer coefficient of the twisted oval tube heat exchanger is higher and the pressure drop is lower than the rod baffle heat exchanger. In order to evaluate the overall performance of the twisted oval tube heat exchanger, a performance evaluation criterion considering both the tube side and shell side performance of a heat exchanger is proposed and applied. The analyze of the overall performance of the twisted oval tube shows that the twisted oval tube heat exchangers works more effective at low tube side flow rate and high shell side flow rate.     

Numerical evaluation of a heat exchanger with inline tubes of different size for reduced fouling rates

This paper describes the numerical evaluation of a novel cross flow tube bundle heat exchanger that combines tubes of different diameter in an inline arrangement for the purpose of reducing gas side particulate fouling rates while preserving acceptable levels of heat transfer and pressure drop performance. Three arrangements are compared: a common inline tube bundle heat exchanger with cylinders of equal diameter and two other arrangements that consist of alternately placed cylinders with a diameter ratio of d/D = 0.5, at two different transverse spacings. Numerical calculations are performed in order to study heat transfer, pressure drop and fouling rates from flue gases with suspended ash particles. The alternating tube sizes achieve a suppression of the vortex shedding mechanism that has previously been shown to enhance downstream particle deposition. Results show that, compared to the standard arrangement, the tube bundle with unequal cylinders placed at the largest transverse spacing achieves a significant (～30%) reduction in particle deposition rate without sacrificing acceptable values of heat transfer per unit volume and low pressure drop.     

Experimental investigation of twisted tape inserts performance on condensation heat transfer enhancement and pressure drop

A comprehensive experimental investigation is conducted on the augmentation of heat transfer coefficients and pressure drop during condensation of HFC-134a in a horizontal tube at the presence of different twisted tape inserts. The test section is a 1.04 m long double-tube counter-flow heat exchanger. The refrigerant flows in the inner copper and the cooling water flows in annulus. The experiments are performed for a plain tube and four tubes with twisted tapes inserts of 6, 9, 12 and 15 twist ratios. The pressure drop is directly measured by a differential pressure transducer. It is found that the twisted tape with twist ratio of 6 gives the highest enhancement in the heat transfer coefficient and the maximum pressure drop compared to the plain tube on a nominal area basis. For this case the enhancement in heat transfer and the pressure drop are increased by 40 and 240% in comparison with to the plain tube. It is observed that the twisted tape with the twist ratio of 9 has the best performance enhancing the heat transfer with the minimum pressure drop. Also empirical correlations are developed to predict smooth tube and swirl flow pressure drop. Predicted results are compared to experimental data and it is found that these correlations are reliable for pressure drop estimation.     

An empirical study on heat transfer and pressure drop characteristics of CuO–base oil nanofluid flow in a horizontal helically coiled tube under constant heat flux

An experimental investigation has been carried out to study the heat transfer and pressure drop characteristics of nanofluid flow inside horizontal helical tube under constant heat flux. The nanofluid is prepared by dispersion of CuO nanoparticle in base oil and stabilized by means of an ultrasonic device. Nanofluids with different particle weight concentrations of 0.5%, 1% and 2% are used. The effect of different parameters such as flow Reynolds number, fluid temperature and nanofluid particle concentration on heat transfer coefficient and pressure drop of the flow are studied. Observations show that by using the helically coiled tube instead of the straight one, the heat transfer performance is improved. Also, the curvature of the tube will result in the pressure drop enhancement. In addition, the heat transfer coefficient as well as pressure drop is increased by using nanofluid instead of base fluid. Furthermore, the performance evaluation of the two enhanced heat transfer techniques studied in this investigation shows that applying helical tube instead of the straight tube is a more effective way to enhance the convective heat transfer coefficient compared to the second method which is using nanofluids instead of the pure liquid.     

多管程微通道冷凝器热力性能计算方法

多管程平行流微通道冷凝器的管程设计方案对换热器管内热力性能影响较大。但目前一直尚未有对其管内换热系数和压降进行理论预测的较为简单可行方法。本文针对各管程工质流量可变,平均干度可变的多管程平行流冷凝器管内热力参数提出一种分程计算方法：在假设管壁温度不变及同管程内流量均匀分配的前提下,采用了Koyama与Wang冷凝换热模型,以及Zhang和Koyama提出的摩擦压降模型,建立了壁温与热流量之间的关系式,通过迭代求得管内平均换热系数和压降的理论值。以一个商用R134a、流程分配为12-8-8-6微通道冷凝器作为示例,用理论和实验方法分别得到了其管内冷凝平均换热系数和压降。结果表明,二者的偏差均落在30%以内。其中Koyama和Zhang 提出的模型预测偏差较小,分别为-4.96%~11.31%,0.42%~25.14%。     

Thermal and hydraulic analysis of a brazed aluminum evaporator

This paper presents an analytical/computer model to predict the performance of a brazed aluminum evaporator operating under dehumidifying conditions. The evaporator uses small hydraulic diameter, flat multi-channel tubes and louver fins. The in-tube refrigerant flow was divided into three regions including the two-phase, liquid deficient and superheat regions. For each region, correlations were selected from the open literature to calculate the local heat transfer and pressure drop. The effects of refrigerant pressure drop along tube and pressure losses at the tube entrance and exit were accounted for in the heat transfer calculations. The air-side fins were assumed to operate at the fully wet condition and the sensible heat transfer coefficient of the wet fins was assumed to be equal to that of the dry fins. The overall heat transfer coefficient was calculated using the enthalpy driving potential method. The total heat transfer rate and refrigerant pressure drop depend on the ratio of the number of tubes in the first and second passes. Parametric studies were done to illustrate selection of the preferred number of tubes per pass. The average refrigerant side heat transfer coefficient is sensitive to the dry-out vapor quality. However, the total heat transfer rate is relatively insensitive to the dry-out vapor quality. As the air inlet humidity increases, the latent and total heat transfer rates increase, but the sensible heat transfer rate decreases. The program was used to design an R-404A evaporator, for which a prototype was built and tested. The program over-predicted the evaporator capacity by 8%. The over-prediction is believed due to flow mal-distribution in the branch tubes.     

椭圆管与扁管管板式换热器换热性能的分析比较

本文对椭圆管与扁管管板式换热器的充分发展的周期性层流流动与换热特性进行了数值计算分析,给出了在400相似文献   

Pressure drop and thermal characteristics of CuO-base oil nanofluid laminar flow in flattened tubes under constant heat flux

An experimental investigation has been carried out to study the heat transfer and pressure drop characteristics of nanofluid flow inside horizontal flattened tubes under constant heat flux. The nanofluid is prepared by dispersion of CuO nanoparticle in base oil and stabilized by means of an ultrasonic device. Nanofluids with different particle weight concentrations of 0.2%, 0.5%, 1% and 2% are used. Copper tubes of 11.5 mm I.D. are flattened into oblong shapes and used as test sections. The nanofluid flowing inside the tube is heated by an electrical heating coil wrapped around it. Required data are acquired for laminar and hydrodynamically fully developed flow inside round and flattened tubes.The effect of different parameters such as flow Reynolds number, flattened tube internal height and nanofluid particle concentration on heat transfer coefficient and pressure drop of the flow is studied. Observations show that the heat transfer performance is improved as the tube profile is flattened. Flattening the tube profile resulted in pressure drop increasing. In addition, the heat transfer coefficient as well as pressure drop is increased by using nanofluid instead of base fluid. Furthermore, the performance evaluation of the two enhanced heat transfer techniques studied in this investigation shows that applying flattened tubes instead of the round tube is a more effective way to enhance the convective heat transfer coefficient compared to the second method which is using nanofluids instead of the base liquid.     

Effect of tube diameter on boiling heat transfer of R-134a in horizontal small-diameter tubes

The boiling heat transfer of refrigerant R-134a flow in horizontal small-diameter tubes with inner diameter of 0.51, 1.12, and 3.1 mm was experimentally investigated. Local heat transfer coefficient and pressure drop were measured for a heat flux ranging from 5 to 39 kW/m², mass flux from 150 to 450 kg/m² s, evaporating temperature from 278.15 to 288.15 K, and inlet vapor quality from 0 to 0.2. Flow patterns were observed by using a high-speed video camera through a sight glass at the entrance of an evaporator. Results showed that with decreasing tube diameter, the local heat transfer coefficient starts decreasing at lower vapor quality. Although the effect of mass flux on the local heat transfer coefficient decreased with decreasing tube diameter, the effect of heat flux was strong in all three tubes. The measured pressure drop for the 3.1-mm-ID tube agreed well with that predicted by the Lockhart–Martinelli correlation, but when the inner tube diameter was 0.51 mm, the measured pressure drop agreed well with that predicted by the homogenous pressure drop model. With decreasing tube diameter, the flow inside a tube approached homogeneous flow. The contribution of forced convective evaporation to the boiling heat transfer decreases with decreasing the inner tube diameter.     

管芯式散热器传热与阻力特性试验研究

鉴于目前汽车管芯式散热器的研究较少,根据JB2293—78试验方法对其进行了传热与阻力试验研究。利用多参数优化拟合计算得出本次试验雷诺数范围内的管芯式散热器试件空气侧对流换热系数和阻力系数的准则关系式。试验结果表明：准则关系式关联精度较好;水管排数对该类型散热器试件换热性能和空气阻力具有较大的影响,四排管时散热器的换热性能最好。     

Multi-Objective Optimization of Turbulent Tube Flows Over Diamond-Shaped Turbulators

In this paper, experimentally derived correlations of heat transfer and pressure drop are used in a Pareto-based multi-objective optimization approach to find the best possible combinations of heat transfer and pressure drop in a tube fitted with diamond-shaped turbulators in tandem arrangements. The design variables are two geometrical parameters of diamond-shaped turbulators, namely, cone angle and tail length ratio, Reynolds number, and Prandtl number. The objectives are maximizing the nondimensional heat transfer coefficient and minimizing the nondimensional pressure drop in a round tube fitted with diamond-shaped turbulators. It is shown that some interesting and important relationships as useful optimal design principles involved in the thermal performance of round tube fitted with diamond-shaped turbulators can be discovered by the Paret- based multi-objective optimization approach.     

Experimental investigation of heat transfer and pressure drop in serrated-fin tube bundles with staggered tube layouts

An experimental investigation of the heat transfer and pressure drop performance of ten finned tube bundles using serrated fins is presented. All tube bundles had staggered layouts, and the influence on varying tube bundle layout, tube and fin parameters are presented. The heat transfer coefficient experienced a maximum when the flow areas in the transversal and diagonal planes were equal. An increase in the fin pitch increased the heat transfer coefficient; the same was observed with an increase in fin height. The pressure drop coefficient showed no influence of the tube bundle layout for small pitch ratios, but dropped significantly for higher ratios. Increasing fin pitch reduced the pressure drop, whereas varying fin height had insignificant effect. None of the literature correlations were able to reproduce the experiments for the entire range of tested conditions. A set of correlations were developed, reproducing the experimental data to within ±5% at a confidence interval of 95%.     

微细管壳式换热器传热性能分析

对微细管管壳式换热器的流动与传热性能进行了实验研究。根据实验获得的微细圆管换热器对流传热努谢尔特数准则式与流动阻力系数的准则式,分析了微细管管壳式换热器的传热流动综合性能,并与传统的管壳式换热器进行了分析对比。结果表明:微细管管壳式换热器传热流动综合传热性能是传统管壳式换热器的2到5倍,且在实验范围内随着雷诺数的增加而增加。     

管排数对三角小翼式涡产生器式管片换热器换热影响的数值分析

通过数值模拟的方法,研究了三角小翼式涡产生器式错排圆管管片换热板芯在1,2,3,4四种不同管排数下的局部对流换热系数以及平均对流换热系数、阻力系数,得出管排数对换热与阻力性能的影响规律,为进一步提高换热器换热性能提供了理论依据。     

Improved Prediction of Shell Side Heat Transfer in Horizontal Evaporative Shell and Tube Heat Exchangers

This paper presents an improved prediction method for the heat transfer and pressure drop in the shell side of a horizontal shell and tube evaporator. The results from an experimental test program are used in which a wide range of evaporating two-phase shell side flow data was collected from a TEMA E-shell evaporator. The data are compared with shell side heat transfer coefficient and pressure drop models for homogeneous and stratified flow. The comparison suggests a deterioration in the heat transfer data at low mass fluxes consistent with a transition from homogeneous to stratified flow. The pressure drop data suggest a stratified flow across the full test range. A new model is presented that suggests the transition in the heat transfer data may be due to the extent of tube wetting in the upper tube bundle. The new model, which also takes into account the orientation of the shell side baffles, provides a vast improvement on the predictions of a homogenous type model. The new model would enable designers of shell side evaporators/reboilers to avoid operating conditions where poor heat transfer could be expected, and it would also enable changes in process conditions to be assessed for their implications on likely heat transfer performance.     

Parametric study and multiple correlations of an H-type finned tube bank in a fully developed region

ABSTRACT

The air-side heat transfer and pressure drop characteristics of an H-type finned tube bank in the turbulent periodically fully developed region are investigated numerically. The effects of seven geometric parameters and Reynolds number are examined. It is found that when the number of tube rows is equal to and larger than 25, the heat transfer and fluid flow approach the fully developed state. Among the seven geometric parameters spanwise tube pitch has the most important effect. Based on the numerical results, the correlations of heat transfer and pressure drop of the H-type finned tube bank for the fully developed region are presented.     

Experimental and numerical study of convective heat transfer and fluid flow in twisted oval tubes

Twisted oval tube heat exchanger is a type of heat exchanger aims at decreasing the pressure drop of the shell side. In the present study, heat transfer and pressure drop performances of twisted oval tube have been studied experimentally and numerically. The experimental study of the twisted oval tube shows that heat transfer process can be enhanced but also with an increasing of pressure drop when compared with the smooth round tube. The effects of geometrical parameters on the performance of the twisted oval tube have been analyzed numerically. The result reveals that the heat transfer coefficient and friction factor both increase with the increasing of axis ratio a/b, while both decrease with the increasing of twist pitch length P. The influence of a/b and P on the overall performance of the twisted oval tubes are also studied. Aiming at obtaining the heat transfer enhancement mechanism of the twisted oval tube, secondary flow, total velocity and temperature distributions of flow section are given. From the analysis it can be concluded that the emergence of twist in the twisted oval tube results in secondary flow. It exists in the form of spiral flow when a/b is big, but in the form of up and down when a/b is small. It is this secondary flow that changes the total velocity and temperature distributions of the twisted oval tube when compared with a smooth oval tube with the same sectional geometric parameters. Then the synergy angle between velocity vector and temperature gradient is reduced and the heat transfer process is enhanced.