Effect of surface etching on condensing heat transfer

This study conducted experiments on humid air condensation during heat transfer in an air preheating exchanger attached to a home condensing boiler to improve thermal efficiency. An etchant composed of sulfuric acid and sodium nitrate was used to create roughness on the heat exchanger surface made from STS430J1L. A counter flow heat exchanger was fabricated to test the performance of heat transfer. Results showed that the overall heat transfer coefficients of all specimens treated with etchant improved with respect to the original specimens (not treated with etchant), and the overall heat transfer coefficient of the 60 s etching specimen increased by up to 15%. However, the increasing rate of the heat transfer coefficient was disproportional to the etching time. When the etching time specifically increased above 60 s, the heat transfer coefficient decreased. This effect was assumed to be caused by surface characteristics such as contact angle. Furthermore, a smaller contact angle or higher hydrophilicity leads to higher heat transfer coefficient.

     

Theoretical and numerical analyses of a ceramic monolith heat exchanger

This study assessed the performance of a ceramic monolith heat exchanger, estimating heat transfer and pressure drop by numerical computation and the ε-NTU method. A heat exchanger consists of rectangular ducts for exhaust gas, a ceramic core, and rectangular ducts for air and exhaust gases, as well as air in the cross-flow direction. The numerical computations were performed for the whole domain, including the exhaust gas, ceramic core, and air. In addition, the heat exchanger was examined using a conventional ε-NTU method with several Nusselt number correlations from the literature to characterize the flow in the rectangular duct. The results of these numerical computation analyses demonstrated that the effectiveness of the heat exchanger, as demonstrated using the ε-NTU method with Stephan’s Nusselt number correlation, came closest to the results of computation with a relative error of 2%. The air-side pressure drops indicated by the results of numerical computation were 13–22% higher than those calculated using the head loss equation with the inclusion of a friction factor that was obtained from previous experiments examining heat transfer conditions.     

燃气钢瓶调质炉余热回收的实践

介绍了一种复合式热管换热器,在高温段采用螺旋槽翅片管换热器,在低温段采用热管换热器进行2级逆流换热,成功地应用于燃气钢瓶调质炉的余热回收利用领域,并讨论了强化传热措施和工业炉燃料的节约率等相关内容。用燃料废气热量来预热空气是燃烧炉最有效的节能措施,例如废气温度为700℃,废气带走的热损失率为40％,若把空气预热到300℃就可以节约燃料达到15％。     

Effects of gas flow on particulate deposition in EGR coolers

This paper presents the test approach to investigate the effects of gas flow rate on particulate deposition inside the exhaust gas recirculation (EGR) coolers. The tests were carried out on a diesel engine equipped with the EGR cooler that a given total gas flow rate passed through and that contained pluggable exchanger tubes to adjust the flow rate in the tubes. Test results show that under the given coolant temperature and EGR gas temperature, the lower flow rate in the exchanger tube is and the longer tubes are, the more particulate with D_p > 50 nm deposit in the cooler. With the given total gas flow rate passing through the cooler, the lower flow rate in the exchanger tube is and the longer tubes are, the larger deposit mass is inside the cooler. In the cases of longer tubes, the effects of gas flow rate on the deposit mass are depends on the effects of gas velocity on the particulate deposition, rather than the effects of flow rate on the particulate number passing through the heat exchanger tube per minute.

     

Experimental investigation of the reverse heat transfer of R134a flow through non-adiabatic coiled capillary tubes

This research represents an experimental investigation of the metastable flow and re-condensation phenomenon through non-adiabatic lateral helical capillary tubes and suction tube heat exchanger. The results show that mass flux ratio has a vital role: It affects metastable flow and also reverse heat transfer phenomenon through non-adiabatic helical capillary tube. Therefore, by increasing of the mass flux ratio, the rate of heat transfer between them decreases. In contrast to the strong rate condition of heat transfer between them, reverse heat transfer or re-condensation maybe happen. Moreover, experimental results show that for R134 flow with mass flux ratio more than 57.84, metastable flow exists in non-adiabatic capillary tube with 0.9144 mm inner diameter, 30 mm coil diameter, 6.18 m length, 4 mm inner diameter of compressor suction tube.

     

Thermal performance analysis of heat exchanger for closed wet cooling tower using heat and mass transfer analogy

In closed wet cooling towers, the heat transfer between the air and external tube surfaces can be composed of the sensible heat transfer and the latent heat transfer. The heat transfer coefficient can be obtained from the equation for external heat transfer of tube banks. According to experimental data, the mass transfer coefficient was affected by the air velocity and spray water flow rate. This study provides the correlation equation for mass transfer coefficient based on the analogy of the heat and mass transfer and the experimental data. The results from this correlation equation showed fairly good agreement with experimental data. The cooling capacity and thermal efficiency of the closed wet cooling tower were calculated from the correlation equation to analyze the performance of heat exchanger for the tower.     

A study on method of reducing condensation for total heat exchanger under humid conditions

An experimental study was performed to reduce the condensation under very humid conditions for a total heat exchanger. The hydrochromic ink was used to detect the condensation within the total heat exchanger. Condensation in a total heat exchanger can be reduced by decreasing the flow rate of the supply air. The other method of decreasing the flow rate of the exhaust air can also be used to reduce condensation. However the flow rate of the exhaust air has to be less than half of the supply air flow rate for the tested total heat exchanger.     

An experimental study on the thermal and fouling characteristics in a washable shell and helically coiled heat exchanger by the Wilson plot method

Brazed plate heat exchangers (BPHEX) are broadly used in water source heat pump systems for their large heat transfer capacity. Despite their high heat transfer rate, their high-performance rate tends to decrease sharply, due to fouling and they cannot be cleaned. So the thermal and fouling resistances of washable Shell and helically coiled tube heat exchangers (SCHEX) are designed and experimentally investigated in this study. Heat exchangers with two different tube types are studied and compared with a brazed plate heat exchanger. The overall thermal resistance coefficient of the heat exchangers as determined by using Wilson plots is 38% lower than that of the brazed plate heat exchanger at a Reynolds number of 2460. Fouling test results revealed that regular maintenance and physical cleaning can be used to maintain the thermal resistance of fouling of the washable heat exchanger at a level equal to or less than that of the brazed plate heat exchanger.

     

Effects of saturation temperature variation due to pressure drop of working fluid in heat exchanger on heat transfer performance

The effect of the saturation temperature drop on the heat transfer performance of a heat exchanger was analyzed under air-conditioner condensing condition, air-conditioner evaporating condition, and refrigerator evaporating condition. The thermodynamic analysis results show that the heat transfer capacity due to the pressure drop of the saturated refrigerant was at least 2.3 % and at most 91.1 % compared to the evaluated heat transfer capacity assuming no pressure loss. The rate of change of heat transfer capacity was the largest in the order of R600a, R1234yf, R134a, R410A, and R32. Heat exchanger performance simulation under practical air-conditioner operating conditions showed that the heat transfer capacity was reduced by 0.72 % due to refrigerant pressure drop under the condensing condition. On the other hand, the heat transfer capacity was increased by 26.55 % under the evaporating condition.

     

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

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.     

Convective heat transfer behaviour of water-ethylene glycol-mixture with silver nanoparticles under laminar flow conditions

In this work, we report the forced convective heat transfer performance and pressure drop of aqueous ethylene glycol seeded with silver nanoparticles for low temperature applications. Experiments were performed in a tube in tube counter-current heat exchanger using silver nanofluid as the hot fluid under laminar flow conditions. In this study, water-ethylene glycol mixture with 70:30 volume percent was used as the base medium. Silver nanofluid was allowed to flow through inner tube of the heat exchanger for varying nanofluid mass flow rates from 5 g/s to 30 g/s and three inlet temperatures of nanofluid viz. 2 °C, 5 °C and 10 °C. The increments in thermal diffusivity and viscosity are found to be ~37 % and ~69 % at 0.45 vol%, respectively. The enhancement in heat transfer coefficient at highest mass flow rate is found to be ~94 % for 0.45 vol%. The pressure drop in the silver nanofluid increases with respect to increase in volume percentage of nanoparticles due to increase in viscosity.

     

Effect of particle ingestion on the fouling reduction and heat transfer enhancement of a No-Distributor-Fluidized heat exchanger

To overcome the fouling problem that is common in heat exchangers for waste heat recovery, a new type of fluidized heat exchanger was devised and tested. Fluidized bed heat exchangers are considered to be a good candidate for waste heat recovery flue gases due to their demonstrated ability to avoid fouling or to clean out deposition on heat transfer surfaces, but have a major drawback with significant pressure losses. These pressure drops typically associated with the distributor plate, which is a key component in constructing any conventional fluidized bed system, limit the applicability of fluidized bed heat exchangers for use as an energy saving device. In a new design, however, dilute gassolid particulate is maintained without having a distributor plate. The main feature of this no-distributor-fluidized (NDF) heat exchanger is the self-cleaning action by ingested circulating particles at minimal additional pressure loss. In the present study, a multi riser NDF heat exchanger of 7,000 kcal/hr capacity was built to evaluate its heat transfer performance and fouling reduction characteristics. To experimentally simulate the fouled condition, fuel rich combustion gas with soot was introduced to the heat exchanger, then a cleaning test was performed by introducing glass bead particles (600μm) inside the gas passage of the heat exchanger unit. Through the present experimental study, the performance degradation due to fouling was successfully demonstrated and the cleaning role of particle circulation was identified. It was also demonstrated that small amounts of circulating particles contribute not only to the fouling reduction on the gas side, but also to the heat transfer enhancement. Experimental operation data for 50 hours including accelerated fouling are obtained to simulate the long-term behavior of the system.     

Boiling heat transfer-based temperature rise characteristics of automotive permanent magnet synchronous motors at peak operating conditions

Using two-phase flow boiling heat transfer theory, the RPI subcooling boiling heat transfer model was established to study the temperature rise characteristics of the permanent magnet synchronous motor (PMSM) of electric vehicles under peak operating conditions, and the effects of coolant inlet temperature, altitude and inlet flow rate on the motor temperature rise were analyzed. The results showed that: the temperature rise characteristics of the motor are closer to the test results when boiling heat transfer is considered after the motor is warmed up, so the effect of boiling heat transfer of the cooling system should be considered when studying the temperature rise characteristics of the motor; The temperature rise characteristic of the motor increases with the increase of coolant inlet temperature at peak working condition. The short time required for the motor winding to reach 150 °C indicates that the motor temperature rises quickly. In the plateau environment, the temperature growth rate of the motor at peak working conditions increases with the increase of cooling water inlet temperature, while the motor temperature decreases with the decrease of atmospheric pressure. Thus, due to the boiling heat transfer phenomenon of cooling water two-phase flow, the temperature rise characteristic of the motor at high altitude is better than that in plain area.

     

Non-absorbable gas effects on heat and mass transfer in falling film absorption

Film absorption involves simultaneous heat and mass transfer in the gas-liquid system. While the non-absorbable gas does not participate directly in the absorption process, its presence does affect the overall heat and mass transfer. An experimental study was performed to investigate the heat and mass transfer characteristics of LiBr-H₂O solution flowing over 6-row horizontal tubes with the water vapor absorption in the presence of non-absorbable gases. The volumetric concentration of non-absorbable gas. air, was varied from 0.17 to 10.0%. The combined effects of the solution flow rate and its concentration on the heat and mass transfer coefficients were also examined. The presence of 2% volumetric concentration of air resulted in a 25% reduction in the Nusselt number and 41% reduction in the Sherwood number. Optimum film Reynolds number was found to exist at which the heat and mass transfer reach their maximum value independent of air contents. Reduced Nusselt and Sherwood numbers, defined as the ratio of Nusselt and Sherwood numbers at given non-absorbable gas content to that with pure water vapor, were correlated to account for the reduction in the heat and mass transfer due to non-absorbable gases in a falling film absorption process.     

Characteristics of fluid flow and heat transfer in a fluidized heat exchanger with circulating solid particles

The commercial viability of heat exchanger is mainly dependent on its long-term fouling characteristic because the fouling increases the pressure loss and degrades the thermal performance of a heat exchanger. An experimental study was performed to investigate the characteristics of fluid flow and heat transfer in a fluidized bed heat exchanger with circulating various solid particles. The present work showed that the higher densities of particles had higher drag force coefficients, and the increases in heat transfer were in the order of sand, copper, steel, aluminum, and glass below Reynolds number of 5,000.     

Swirl flow post-CHF heat transfer with refrigerant 113

Heat transfer rate was experimentally determined in the post-CHF region of a steady-state two-phase flow of a refrigerant in a vertical tube with swirl induced by twisted-tape inserts. Experiments were performed with the vertical flow of refrigerant-113 in a tube with inside diameter of 7.75 mm, a heated length of 3.66 m and mass flux of 375–535 kg/m²s for swirl flow at a pressure of 0.184 MPa. Four tapes were used with twist-ratio of 2.5 to 9.2 for swirl flow. Liquid heating produced the low wall-superheat in the post-CHF region at steady-state, which is typical of heat exchanger operation. Superheated vapor measured at the test section exit in most tests ensured that entire post-CHF region was included. All refrigerant-113 data were compared with the data of water and refrigerant-12. The existing post-CHF heat transfer correlation of swirl flow was modified to predict the magnitude and trends of the data of the three fluids such as water, R-12 and R-113.     

A thermal design method for the performance optimization of multi-stream plate-fin heat exchangers

An optimization design method based on field synergy principle is developed for Multi-stream plate-fin heat exchangers (MPHEs) with a segmented differential model. The heat exchanger is divided into a number of sub-exchangers along the main stream, and each sub-exchanger consists of N passages along the height of the exchanger. Compared with the traditional heat exchanger design, this method allows temperature and pressure fields to be obtained via coupling calculation with consideration of variable physical properties and the axial heat loss of the heat exchanger. Finally, the heat exchanger is optimally designed using a temperature-difference uniformity optimization factor based on field synergy principle. This design model can provide an accurate temperature field and pressure field, because the stream properties are determined by the mean temperature and pressure of each local sub-exchanger. Optimum results indicate that the temperature distribution on the cross section of the heat exchanger is relatively uniform and that the temperature difference of heat transfer for each stream is always a small value. These characteristics prove the feasibility and effectiveness of this design model. In this paper, a case of five stream plate-fin heat exchangers for an ethylene plant is designed under a practical cold box operating condition with the proposed model, the structure and heat transfer of which are optimally determined. The design model and optimization method proposed in this work can provide theoretical and technical support to the optimization design of MPHEs.

     

铜翅片管热水器的特性研究

以铜翅片管换热器为研究对象,通过合理的简化模型,以相邻2片换热片(取半片)及其之间的烟气通道作为计算区域,利用流体力学软件CFX对烟气通道的流动状况和换热情况进行了数值模拟。研究了管径、翅片厚度和翅片间距对排烟温度的影响,结果表明随着管径和翅片间距的增大排烟温度升高,随着翅片厚度增大排烟温度降低,排烟温度的升高或降低将影响排烟损失的大小,从而影响热效率。该数值模拟结果对改进翅片结构以提高热效率有一定指导意义。     

CO_2热泵热水器的系统设计及试验研究

设计搭建了蒸发器和气冷器均采用套管式换热器的跨临界CO2热泵热水器性能测试试验台,在制冷剂充注量1.23kg时,通过调节膨胀阀的开度和控制气冷器的水流量来研究系统性能。结果表明:该机组能在较高COP(3.2)下制得65℃的热水,并可以在COP不低于2.0情况下制取80℃的热水;气冷器水流量对系统的COP、出水温度以及系统的排气压力影响最大;高效的换热器可以在压缩机排气温度一样的情况下提高出水温度,使系统在制取高温水时有更高的COP。     

Experimental investigation on convective heat transfer and friction factor in a helically coiled tube with Al2O3/water nanofluid

In this study, the heat transfer and friction factor of a shell and helically coiled tube heat exchanger using Al₂O₃ / water nanofluid at 0.1%, 0.4% and 0.8% particle volume concentration were tested. The test was conducted under laminar flow condition at 5100 < Re_i < 8700. It is found that the overall heat transfer coefficient, inner heat transfer coefficient and experimental inner Nusselt number are 24%, 25% and 28%, respectively, higher than water at 0.8% particle volume concentration of nanofluid. It is observed that the presence of nanoparticles further intensify the formation of secondary flow and proper mixing of fluid when nanofluid passes through the helically coiled tube. Apart from further flow intensification, higher thermal conductivity of nanofluid and random movement of nanoparticles contribute to the enhanced heat transfer coefficient. Also found that the friction factor increases over particle volume concentration and this is due to increased nanofluid viscosity while increasing particle volume concentration.