Tube Heat Research Articles

Numerical Studies on Hydrogen Production from Ammonia Thermal Cracking with Catalysts

To explore and optimize the process of hydrogen production from plasma-assisted ammonia-cracking, a tubular ammonia-cracking on-site hydrogen production device with plasma-assisted ammonia combustion flue gas as the heat source was developed. Using the Temkin–Pyzhev kinetic model and the local thermal equilibrium (LTE) hypothesis, the effects of operating conditions, such as combustion flue gas temperature and ammonia flow rates, on ammonia-cracking efficiency were investigated. The numerical results are quantitatively consistent with the experiment. Ammonia cracking efficiency is notably influenced by the initial combustion gas temperature. When the gas velocity of the cracking system is less than or equal to 0.03 m/s, the cracking rate increases by 63% when the inlet temperature of the heat pipe changes from 700 K to 800 K. The cracking rate of ammonia decreased with the increase of ammonia flow rate, and this trend reached the maximum and began to weaken when the flow rate was 0.3 m/s. Longer catalyst bed length does not always mean higher cracking efficiency; the length of the cracking tube over 0.6 m shows little effect on cracking efficiency. Response surface methodology was used to conduct multi-factor analysis of the three main factors affecting the cracking rate of the cracker, namely, the temperature of the heating tube, the flow rate of flue gas in the heating process, and the inlet flow rate of the catalytic bed. It was found that the flow rate of the catalytic bed was the most significant factor affecting the cracking rate, which could be used as the main control method. The numerical results would provide technical guidance for industrial applications of on-site hydrogen production devices from ammonia decomposition.

Cheetah optimization algorithm for optimum design of heat exchangers

Abstract Thermal system optimization is always a challenging task due to several constraints and critical concepts of thermo-hydraulic aspects. Heat exchangers are one of those devices that are widely adopted in thermal industries for various applications such as cryogenics, heat recovery, and heat transfer applications. According to the flow configurations and enhancement of fins, the heat exchangers are classified as plate-fin heat exchangers, shell and tube heat exchangers, and tube-fin heat exchangers. This article addresses the economic optimization challenge of plate-fin heat exchangers using cheetah optimization (CO) algorithm. The design variables were optimized using the CO algorithm, and statistical results were compared with eight well-established algorithms. The study revealed that the cheetah algorithm is prominent in terms of realizing minimizing the overall cost of the plate-fin heat exchanger with a 100 % of success rate. Furthermore, the study suggests adopting the cheetah optimizer for solving optimization challenges in different fields.

Effect of flue gas constituents on boiler tube failure of a captive power plant

Water leakage was reported from one of the superheater tubes of boiler during operation. The power plant is a part of copper ore smelting and refining industry. It was observed, that the leakage took place through cracked region of the tube, which was severely damaged due to chemical attack. Damage occurred due to reaction between flue gas and outer wall of super heater tube. Inefficient treatment of flue gas resulted in retention of metal particles, SO2 gas and particulate within the same in substantial concentration. When the same flue gas was used in boiler, SO2 reacted with tube surface at elevated temperature. The incident lead to wall thinning and deep pit formation over tube surface. Metal particles was fused, adhered to tube surface and triggered secondary cracking during operation. Gradual wall thinning, increment in pit size and crack propagation through thickness resulted in ultimate failure of tube material.

Flat micro heat pipe-based shell and tube storage unit for indirect solar dryer: a pilot study.

Poria cocos has been dried in an indirect solar drying system composed of a roughened solar air heater (RSAH), a shell and tube storage unit assisted with flat micro heat pipes fins, and a drying chamber. The main novelty in this study is using FMHPs as fins in shell and tube storage unit with paraffin wax and lack of investigations on Poria cocos solar drying as medicinal material used in Chinese medicine. First and second laws of thermodynamics are used to assess the performance of the system and the results indicated that the RSAH average thermal ( ) and exergy efficiency ( ) were 73.9% and 5.1%, respectively, with averaged incident solar radiation of 671 W/m2 under airflow rate of 0.0381 m3/s. Furthermore, the storing system showed 37.6% as averaged overall and 17.2% as averaged overall , as well as, discharging prolonged to 4h with effective drying temperature. The overall of the dryer was 27.6% with specific energy consumption (SEC) of 8.629 kWh/kg moisture. The payback period of the system is 1.7years.

Prediction of heat transfer coefficients for steam condensation in the presence of air based on ANN method

Artificial neural network (ANN) methods have been gradually used in the field of nuclear reactor thermal-hydraulics as new methods to improve accuracy or fast prediction. This study establishes a back propagation (BP) neural network model based on the ANN methods to predict the steam condensation heat transfer coefficient outside a heat tube in the presence of air. The main factors affecting condensing heat transfer, such as pressure, air mass fraction, subcooling, and tube diameter, are used as input quantities, and the condensation heat transfer coefficient is used as output quantity. A complete set of neural networks for predicting the heat transfer coefficient for steam condensation in the presence of air is established based on the relevant experimental data collected over the world. The results predicted by the ANN model are compared with the experimental data and those of traditional correlation methods. The data from 2276 experiments are distributed within a ±10% error band at the 95% confidence level. This means that the prediction accuracy of the ANN model is higher than that of the traditional experimental correlation. Therefore, the neural network model developed in this study can be used for the prediction of the heat transfer coefficient for steam condensation in the presence of air.

A novel structure tube for supercritical CO2 turbulent flow with high non-uniform heat flux

Supercritical CO2 has been a promising alternative working medium in coal-fired power plants, high-temperature solar power systems, and fuel cells. In these cases, supercritical carbon dioxide is in a round tube under high temperature and pressure with high non-uniform heat flux. In this study, a novel structure tube is proposed by optimizing the coupling between circumferential heat flux and tube thickness on the heat-absorbing side to improve its comprehensive performance. A thermal-fluid-mechanical coupling model was developed. The novel structure can reduce maximum temperature and temperature difference effectively for the half-cycle uniform and non-uniform heat flux. The maximum thermal stress decreased by 23 and 29% and the maximum temperature decreased by 29.6 and 24.8 K for half-cycle uniform and non-uniform heat flux when the Eccentricity increased from 0 to 0.4. A comparison was made between the proposed structure, thin-walled tube, and traditional tube. Results showed that the performance of the proposed tube is the best. The maximum temperature decreases by approximately 55 K compared with the traditional and thin-walled tubes. The maximum equivalent stress of proposed tube is the smallest. Furthermore, the structure will improve the safety and economics of the tube.

Effect of tubular heater structures on the thermal behaviors of waxy crude oil

In this paper, we research the temperature field, flow field and melting law of gelled crude oil with different diameters, locations and inclination angles of heating tube by numerical simulation method. Using the synergy angle, temperature variance and effective heating ratio as evaluation indices, the effect of the heating tube and distribution homogeneity of the temperature field were examined. The simulation results show that the temperature field and velocity field of waxy crude oil and the melting law of gelled crude oil are basically the same under different conditions. The most important factor affecting average temperature and temperature variance is the location of heating tube, which can increase the average temperature by 1.252 °C and decrease the temperature variance by 9.019°C2. The most important factor affecting the synergy angle is the inclination angle of heating tube, which can be reduced by 4.104°. In the 8 cases, when the inclination angle of the heating tube is 45°, the temperature maintaining effect of tubular heating is the best, and the synergy angle is the smallest. The least temperature variance is the uniform arrangement of heating tube. The results of this study will provide theoretical guidance for the optimization design of tubular heater structure.

A numerical study of internally heating, counter-flow tubular packed bed reactor for methanol steam reforming

This study numerically investigates methanol steam reformer design using the COMSOL software. The reformer is heated with internal heating tubes (HTs). The effects of the reformer shape, the heating tube arrangement and heating rate are studied. Results show that a divergent reformer shape has the highest methanol conversion (68%) due to slower average flow rate of the reactants, longer residence time, and more heat absorption. For fixed heating rate, increasing the number of HTs increases the methanol conversion rate by up to 14.8% due to better heat transfer. Furthermore, methanol conversion can also be enhanced with increasing heating rate and placing HTs towards reactor center. The optimal design of a divergent tube with a divergent angle of 5° and 8 HTs has a maximum conversion rate of 94.5%. The results of this study are helpful for engineers working on designing an MSR with high-methanol conversion rate.

Thermal decomposition of polypropylene plastics through vacuum pyrolysis

The technique of thermal pyrolysis has a great contribution to waste-to-energy platform and circular economic mission. This study used an internal heating process for the vacuum pyrolysis of plastic waste. Eight heating tubes were placed inside a reactor to accelerate heat transfer to the feedstock. Uniform temperature distribution was achieved in <1 h. After 90 min, three phases of pressure profiles were identified: decomposing samples, maximum feedstock cracking, and stability occurrences. Internal pressure decreased with the reduction of condensable gas. Pyrolytic oil was similar to conventional fuels (gasoline and diesel), with the percentage of lower hydrocarbons C5–C10 at 73%, C11–C22 at 18%, and >C22 at 9%. Gas chromatography–thermal conductivity detector analysis of noncondensable gases revealed that the main product of PP with 75% area was C2H6. Furthermore, the Taguchi method was used to optimize the vacuum pyrolysis of waste to energy. Optimal operating conditions for maximum pyrolytic yields were as follows: temperature 425 °C, heating rate 15 °C/min, residence time 6 h, and output flow rate 0.1 L/min. Moreover, the physical properties, cetane number, soot propensities, and auto-ignition temperature of pyrolytic oil were comparable to petroleum diesel. The heating value of the oil product was 55.96 MJ/kg, which was higher compared with petroleum fuels. Vacuum pyrolysis mass, carbon, and energy balances were also determined. This study demonstrated the novel use of internal heating in vacuum pyrolysis. Ultimately, this study provided a new technique of internal heating method in vacuum pyrolysis.

Effect of mechanical stirring on the tubular heating process of crude oil

Research on thermal behaviors under the synergy of mechanical stirring and tubular heating can reveal the effect mechanism of mechanical stirring, and determine the optimal stirring parameters. In this paper, the physical and mathematical models of heat transfer in crude oil storage tank under the synergy of tubular heating and mechanical stirring are established, the sliding grid technique is used to model the stirring impeller, which are further numerical calculated by the FVM. Our outcomes demonstrate that through the enhancement of convection, mechanical stirring significantly changes the thermal behaviors and enhances the heat exchange. Precisely, for a 1,000 m3 crude oil storage tank, the stirring of 350 rpm can increase the average velocity by 329%, the heating rate by 89%, and rise the average crude oil temperature from 42.311 °C to 42.586 °C during 2 h of heating. However, the higher crude oil temperature will cause a 1,613 W increase in heat loss power. The heating efficiency has also increased by 10%. Furthermore, mechanical stirring can promote a more uniform temperature distribution. Thus, the volume ratio in the low-temperature region decreases by 10% and that in the high-temperature region increases by 26%. The temperature variance decreases from 4.517 °C2 to 0.275 °C2. However, the velocity and temperature fields are almost the same under different stirring rates. With increasing stirring rate from 100 rpm to 700 rpm, the average heating rate can only increase by 0.026 °C/h, and temperature variance decreases from 0.596 °C2 to 0.086 °C2. With increasing stirring rate, the efficiency of the heating tube does not change significantly. The optimal stirring rate is determined as 350 rpm based on the fitting curve of temperature variance stabilization time and the comprehensive effect.

Numerical Simulation of the Ca(OH)2/CaO Thermochemical Heat Storage Process in an Internal Heating Fixed-Bed Reactor

Using a Ca(OH)2/CaO thermochemical heat storage system is an effective way to promote the utilization of renewable energy. However, poor thermal conductivity restricts the application of a widely used fixed-bed reactor. To improve the heat storage rate, the internal heating mode, which heats the reactant via the internal heating tube instead of the external wall, was adopted, and the heat storage process in the fixed-bed reactor was investigated numerically. The results show that the number and location of tubes have a significant impact on heat storage performance. Compared with the external wall heating mode, the optimized scheme of six internal heating tubes can shorten the reaction time by 21.78%. The temperature and reaction extent distribution reveal that as the reaction proceeds, the optimized scheme has a higher temperature and reaction extent. Additionally, the effects of different conditions, such as solid particle porosity, wall temperature, outlet pressure, and solid particle size, were also analyzed. The study demonstrates that increases in solid particle porosity, wall temperature, and solid particle size as well as a decrease in outlet pressure can improve the heat storage rate.

Effects of different working gases on the heat transfer enhancement performance of a heating tube with spiral insert under oscillatory flow

The working gas type had an important effect on the heat transfer performance of the turbulence device in the heater of Stirling engine, but the related researches were scarce, especially under the oscillatory flow. This work investigated the effects of four working gases (i.e., H2, He, N2, and CO2) on the heat transfer characteristics of a heating tube with spiral insert compared with a smooth tube under oscillating flow for a Stirling engine. The transient physical fields under different phase angles in an oscillatory cycle were analyzed, and the Nusselt (Nu) number, pressure loss and outlet temperature of working gas were studied. The results show that the cycle average Nu number of the enhanced tube with H2, He, N2, and CO2 as working gas increased to 1.67, 1.62, 1.61 and 1.72 times when comparing with those of the smooth tube. The cycle average friction coefficient increased to 1.96, 2.37, 2.36 and 2.62 times, respectively. Moreover, the performance evaluation criterion (PEC) values of the enhanced tube using the four types of working gas were all greater than 1 (1.21∼1.33). This implies that the comprehensive heat transfer performance of the heating tube with spiral insert was all improved with the four types of working gas. Moreover, the heat transfer enhancement effect was best when hydrogen was used. While considering the thermodynamic performance and safety reliability, the helium was more recommended. The findings are beneficial to enhance the operating efficiency of a Stirling engine.

NUMERICAL SIMULATION STUDY ON THE REACTION PERFORMANCE OF A METHANOL STEAM REFORMING TO HYDROGEN MICROREACTOR

Hydrogen has received widespread attention as a new clean energy in order to reduce the carbon emissions of fuel vehicles. This paper studies a tubular microreactor based on methanol steam reforming. Methanol and steam are mixed in proportion and the chemical reaction takes place in a porous catalytic bed. For heating purposes, hot gas from the burner penetrates the reactor bed through heating tubes. Energy is supplied through the heating tubes to drive the endothermic reaction system. The microreactor is enclosed in an insulated jacket. In this paper, parameters such as methanol conversion and hydrogen concentration are evaluated by considering microreactor materials, heating gas temperature and flow direction, heating tube distribution, pressure drop and reaction channel length. First of all, choosing a microreactor material with a smaller thermal conductivity can avoid excessive heat loss, and improve heat transfer performance. Increasing the heating gas temperature leads to an increase in the temperature of the reaction zone, thereby increasing the CH3OH conversion rate and H2 mass fraction. Changing the flow direction of the heating gas affects the reaction rate, but has little effect on the reaction result. Through the research on the distribution of the heating tubes, the results show that the hydrogen production rate is higher when the contact area between the heating tubes and the reaction zone is larger. Secondly, through the comparison of the data under different pressure drops, the best parameter [Formula: see text][Formula: see text]pa is obtained, and the CH3OH conversion rate is 80.6% at this time. Finally, increasing the length of the reaction channel can make the reaction more complete. For example, when the reaction channel length [Formula: see text][Formula: see text]m, the CH3OH conversion rate is as high as 83.7%.

Numerical simulation on heat transfer enhancement of free piston Stirling engine heater with composite cross section mini channels

Integral fin with micro-channels have been widely used in small free piston Stirling engines heater due to their high heat transfer rate, low flow loss, and high machining accuracy. A β-type free piston Stirling engine heater with composite cross section mini channels(CCSMH) is designed for heat transfer enhancement applying to solar dish power generation system. Investigation on heat transfer performance of the heater under oscillating flow is carried out by CFD method, which is compared with smooth annular tube heater. The influence of geometry parameters on the performance of heat transfer and pressure loss is investigated. A improved performance evaluation criteria PEC is proposed for the condition of equal flow velocity to evaluate the enhanced heat transfer performance of the heater. The results show that the maximum value of the comprehensive performance evaluation index PEC can reach 3.14, the maximum mean pressure drop increment of CCSMH compared with the smooth tube heater is 38.4 Pa, which is much lower than the minimum pressure of 2.3 MPa and can be neglected.

Behavioural Investigation of Hardness Profile for Dissimilar TIG-Weldment between P22 and P91 Steels

The Gas Tungsten Arc Welding (GTAW) is now commonly being used to join condenser pipes of railway locomotives, beside boiler and super heater tubes, generally made from dissimilar steel-alloys like P22 (2.25Cr-1Mo) and P91 (9Cr-1Mo). After welding, excessive hardness values of the Heat Affected Zones (HAZs) of those dissimilar weld joints are certainly obtained due to uneven and uncontrolled cooling rates, which resulted in an eruption of abrupt martensitic transformations. This ultimately leads to cracking and failure while operating at elevated temperatures. Therefore, there is a dire need to establish an appropriately optimized post-weld heat treatment process to eliminate this hazardous nuisance. The present work statistically studies the hardness profile of these critical material zones, before and after GTAW.

Performance assessment of an evacuated tube solar-electric hybrid dryer for lotus seeds drying: Moisture removal behavior, GHG emission and thermodynamic analysis

A novel evacuated tube solar-electric hybrid dryer (ETSED) was developed and evaluated to contribute to global food security from the perspective of optimized processing while reducing energy consumption and greenhouse gas (GHG) emissions. The performance of the ETSED was assessed compared with solar heating drying (SHD), and mixed heating drying (MHD) at temperatures of 55, 65 and 75 °C by studying the drying behavior of the samples as well as the thermodynamic analysis and GHG emissions of the system. Results revealed that under all studied experimental conditions, the evacuated tube solar collector provided more than 60% of the thermal supply in the drying process, and significantly improved the fluid exergy. Moreover, the roller shutter mechanism and the heating tube were used together to achieve precise control of the drying temperature. In terms of thermodynamic analysis, the heating tube further enhanced the ETSED performance by providing high quality energy in combination with solar energy in MHD mode. Furthermore, compared with SHD, MHD-65 and 75 °C shortened the drying time by 10.4 and 28.2%, improved the drying efficiency by 0.7% and 6.29%, and increased the drying chamber's exergy by 14.99 and 19.36%, respectively. Remarkably, the lowest emissions of CO2 (872.06 g/kg removed water), SO2 (0.13 g/kg removed water) and NOx (0.19 g/kg removed water) were seen in the case of SHD. Based on the obtained results, the increased drying temperature in the MHD led to higher CO2, SO2 and NOX emission levels. Interestingly, when the lotus seeds were dehydrated, the accumulated electric energy generated by the photovoltaic cells almost met the electric consumption of mechanical energy. The results of this study identified the potential of ETSED equipment for food industrial production.

Improved heating floor thermal performance by adding PCM microcapsules enhanced by single and hybrid nanoparticles

AbstractA heating floor is a low‐temperature emitter consisting of pipelines in which a fluid circulates between 35°C and 45°C. To ensure energy efficiency, occupant comfort, and building material durability, proper heat management is crucial in buildings. By using phase change materials (PCMs) in building envelopes, the indoor temperature can be regulated through the storage and release of thermal energy, which reduces energy consumption and enhances occupant comfort. In this study, we evaluated numerically a heating floor that incorporates a PCM enhanced by nanoparticles (NePCM). The aim of the numerical analysis is to assess the impact of the addition of single and hybrid nanoparticles in different proportions to the PCM layer on the thermal performance of the PCM‐based floor. Therefore, two main objectives are defined. The primary is to take advantage of the storage capacity of a PCM layer by integrating it into the ground; second, to evaluate the hot water temperature levels effect on the floor's performance. Additionally, we address the low thermal conductivity of PCM by enhancing PCM microcapsules with single and hybrid nanoparticles and comparing them to pure PCM. The numerical results obtained show that positioning the PCM microcapsules above the heating tubes (upper position) provides an optimum improvement in thermal performance. Moreover, the addition of hybrid nanoparticles within the base PCM, 1% of Cu mixed with 4% of Al2O3, allows an increase of 4°C, which relates to a reduction of 18% in the internal temperature amplitude and a phase shift of 6 h 30 min compared with the conventional heated floor in which there is no PCM.

Comparative heat transfer analysis on Fe3O4–H2O and Fe3O4–Cu–H2O flow inside a tilted square porous cavity with shape effects

The objective of this paper is to investigate the energy transmission rate's features of the natural convective radiative Fe3O4–H2O nanofluid and Fe3O4–Cu–H2O hybrid nanofluid flow in a tilted square porous cavity under the influence of heat source/sink. The marker and cell method is adopted to solve the system of partial differential equations. The outcomes explore that in the existence of heat source, by augmenting the volume fraction of spherical-, cylindrical-, column-, and lamina-shaped nanoparticles from 1% to 5% in water, the average heat transfer rate is boosted by 6.07%, 8.36%, 9.89%, and 14.95%, respectively. In the existence of heat sink, the increment is noticed as 2.67%, 3.68%, 4.37%, and 6.64%, respectively. Therefore, the shape of the nanoparticles considerably varies the heat transfer rate. In the existence of heat source, by magnifying the volume fraction of spherical-, cylindrical-, column-, and lamina-shaped Fe3O4–Cu nanoparticles from 1% to 5% in water, the mean heat transfer rate is magnified by 7.23%, 11.03%, 14.15%, and 31.36%, respectively. In the existence of heat sink, the magnification is detected as 3.18%, 4.87%, 6.27%, and 14.09%, respectively. This result confirms that the proper combination of nanoparticles considerably enhances the heat transfer characteristics of base fluids. The findings of this study may be helpful for a better understanding of hydrothermal features of thermal systems such as heat exchangers, helical heat sinks, solar collectors, periodic pin-fins, mini shell and tube heat exchangers, plate evaporators, photothermal cancer treatment, and microvascular vessels using various unitary and hybrid nanofluids.

Review of the impact of nanomaterial on the thermal efficiency of an evacuated tube solar air heater

The purpose of this review article is to analyse the use of nanomaterials to augment the thermal efficiency of an evacuated tube solar air heater (ETSAH). As a consequence of this, different varieties of nanomaterial are developed and utilized in the evacuated tube solar air heater. A comprehensive investigation is being carried out into both the incorporation of nanomaterials into the evacuated tube collector and the improvement in performance. During the course of this in-depth research, a number of issues have been investigated. One of these is how the enhancement of heat transfer is affected by the use of distinct nanofluid types, sizes, volume fractions, and applications in a variety of evacuated tube collector types. For the purpose of justifying and, ultimately, explaining the behaviors of nanomaterials for the purpose of thermal augmentation of evacuated tube collector, analyses have been performed and are being addressed here. Incorporating nanoparticles, as shown by an analysis of the relevant literatures, results in a substantial improvement in the thermal efficiency. From the literature survey, the heat pipe evacuated solar air heater tube using SWCNTs nanofluids was found to have a maximum enhanced efficiency of 93.43%.

Household solid desiccant dehumidifier with multiple operating modes: Concept and prototype device

Solid desiccant dehumidification has proven to be more energy-efficient than traditional condensation dehumidification. However, owing to the complexity and large size of existing solid desiccant dehumidifier systems, their application in residential spaces is limited. In this study, we explore the potential of using small solid desiccant dehumidifiers in residential spaces. The dehumidifier uses a variable frequency fan to replace the two fans of a traditional solid desiccant dehumidifier. Meanwhile, the flow direction of the dehumidified air and regeneration air was adjusted via the air valve such that the air in different working modes flows in the same space, thereby reducing the size of the dehumidifier. Moreover, the dehumidifier used an equalizing orifice to ensure the stability and uniformity of the airflow in a small space and used an electric heating tube for regeneration, as opposed to a heat exchanger, to reduce the size of the system. The experimental results of the prototype device show a dehumidification efficiency of more than 85%. The humidity at the inlet of the dehumidifier, air volume, and adsorption heat affected dehumidification performance. The analysis results of the energy coefficient of dehumidification (ECOD) index indicated that through reasonable design and usage patterns, the ECOD of small-sized solid desiccant dehumidifiers can exceed 1.73 kg/(kW·h). Compared to traditional condensing dehumidifiers for home use in the market, the energy efficiency ratio is improved by 8.1%. This indicates that the dehumidifier exhibits a high energy efficiency ratio.