Parameter optimization through performance analysis of model based control of a batch heat treatment furnace with low NOx radiant tube burner

A model based control structure for heat treating a 0.5% C steel slab in a batch furnace with low NO_x radiant tube burner is designed and tested for performance to yield optimal parameter values using the model developed in the companion paper. Combustion is considered in a highly preheated and product gas diluted mode. Controlled combustion with a proposed arrangement for preheating and diluting the air by recirculating the exhaust gas that can be retrofitted with an existing burner yields satisfactory performance and emission characteristics. Finally, the effect of variable property considerations are presented and critically analyzed.     

Radiant and convective heat transfer for flow of a transparent gas in a short tube with sinusoidal wall heat flux

The present analysis accounts for combined convective and radiant heat transfer to a fluid flowing in a short tube with prescribed wall heat flux. The heat flux distribution used was of sine shape with maximum at the middle of the tube. This solution is known to represent the axial power variation in a nuclear reactor core. The tube wall and gas bulk temperatures were obtained by successive substitutions for the wall and gas energy balance equations. The integrals were approximated by Sympson's rule and initial guesses for the iterative process were based upon limiting cases for pure radiation and pure convection. The results of the combined solution compared with the pure radiation approach show a decrease of 30 percent for the maximum wall temperature using black surface ($\text{ε=1}$). For this same situation, the increase in the gas temperature along the tube shows a reduction of 58 percent when compared to the pure convection solution.     

Cocurrent turbulent mixed convection heat and mass transfer in falling film of water inside a vertical heated tube

A detailed numerical study has been conducted in order to analyse the combined buoyancy effects of thermal and mass diffusion on the turbulent mixed convection tube flows. Numerical results for air-water system are presented under different conditions. A low Reynolds number k-ε turbulent model is used with combined heat and mass transfer analysis in a vertical heated tube. The local heat fluxes, Nusselt and Sherwood numbers are reported to obtain an understanding of the physical phenomena. Predicted results show that a better heat transfer results for a higher gas flow Reynolds number Re, a higher heat flux q_w or a lower inlet water flow Γ₀. Additionally, the results indicate that the convection of heat by the flowing water film becomes the main mechanism for heat removal from the wall.     

Numerical study on coupled fluid flow and heat transfer process in parabolic trough solar collector tube

A unified two-dimensional numerical model was developed for the coupled heat transfer process in parabolic solar collector tube, which includes nature convection, forced convection, heat conduction and fluid-solid conjugate problem. The effects of Rayleigh number (Ra), tube diameter ratio and thermal conductivity of the tube wall on the heat transfer and fluid flow performance were numerically analyzed. The distributions of flow field, temperature field, local Nu and local temperature gradient were examined. The results show that when Ra is larger than 10⁵, the effects of nature convection must be taken into account. With the increase of tube diameter ratio, the Nusselt number in inner tube (Nu₁) increases and the Nusselt number in annuli space (Nu₂) decreases. With the increase of tube wall thermal conductivity, Nu₁ decreases and Nu₂ increases. When thermal conductivity is larger than 200 W/(m K), it would have little effects on Nu and average temperatures. Due to the effect of the nature convection, along the circumferential direction (from top to down), the temperature in the cross-section decreases and the temperature gradient on inner tube surface increases at first. Then, the temperature and temperature gradients would present a converse variation at θ near π. The local Nu on inner tube outer surface increases along circumferential direction until it reaches a maximum value then it decreases again.     

Combined mixed convection and radiation heat transfer in rectangular ducts rotating about a parallel axis

The study of combined heat transfer of convection and radiation in rectangular ducts rotating in a parallel mode was investigated numerically in detail. The coupled momentum and energy equations are solved by the DuFort–Frankel numerical scheme to examine the interactions of convection with radiation. The integro-differential radiative transfer equation is solved by the discrete ordinates method. Results are presented over a wide range of the governing parameters. The present results reveal that the rotational effect in a square duct is more significant than that in a rectangular one. The predictions also demonstrate that the radiation presents significant effects on the axial distributions of the total Nusselt number, Nu_t, and tends to reduce the centrifugal-buoyancy effects. The effect of rotation on the Nu_t is restricted in the entrance region, however, the radiation affects the heat transfer through out the channel. Additionally, the Nu_t increases with the decrease in the conduction-to-radiation parameter N_C.     

Relationships between the properties of an urban street canyon and its radiant environment: Introduction of appropriate urban heat island mitigation technologies

The relationships between the properties of urban canopy components and the radiant environment in an urban street canyon are examined considering the introduction of appropriate urban heat island mitigation technologies. Radiant heat transfers between walls and roads are calculated according to Gebhart’s radiant absorption coefficients and using the Monte Carlo method. Roads are classified as either north–south or east–west; intersections are also considered. The key property of an urban street canyon is expressed by its aspect ratio W/H. A simple street canyon model and two actual urban street canyon areas are used as the objects of examination. Distributions of surface temperatures and solar radiation gains on street canyon roofs, roads, and walls are analyzed. The top priority for the implementation of urban heat island mitigation measures concerns the buildings with large roof areas. The other high-priority areas for implementing mitigation measures focus on smaller roofs and roads for which the street canyon aspect ratio W/H is greater than 1.5; the lowest-priority area is the walls.     

Development of a high efficiency radiation converter using a spiral heat exchanger

Heating by radiation is widely used for materials processing. Electrical radiant heaters are the most commonly used heaters. Electricity is expensive and the combustion of fossils fuels for electricity production emits CO₂. In order to convert the energy from the fuel to radiation energy directly and efficiently, our group has developed a compact, high efficiency, radiation converter using a spiral heat exchanger to recover the energy from high-temperature exhaust gas. The spiral heat exchanger has a weld-free construction to prevent cyclic thermal stress, and is constructed from inexpensive ferrite steel plates. The combustion chamber, equipped with a swirler to mix the gas fuel and air, can achieve stable combustion. The distribution of the surface temperature on the radiant tube was measured by a radiation thermometer, called a thermo viewer, and then the radiant energy emitted from the radiant tube was estimated. The efficiency of the spiral heat exchanger was measured from the temperature of the inlet air and exhaust gas. The heat exchanger achieved a high effectiveness, and heat loss from the exhaust gas was minimized. Consequently, a highly efficient radiation converter was produced to convert the fuel energy to radiation energy.     

Influence of helical tube dimensions on open channel natural convection heat transfer

Natural convection heat transfer of a helical tube was investigated experimentally for varying tube diameter, length, height, pitch, radius, and number of turns, in order to determine an appropriate characteristic length to describe the phenomenon. Mass-transfer rates of a CuSO₄–H₂SO₄ electroplating system were measured by replacing the heat transfer system according to the analogy concept. When the pitch-to-diameter ratio was larger than 5 and the pitch-to-radius ratio was smaller than 2.3, the heat transfer rates were very close to those of a horizontal cylinder, and decreased with the diameter of the tube while remaining unaffected by the total length and height. The natural convection heat transfer of the Nth turn of a helical tube was measured for varying pitch-to-diameter ratio and number of turns, and the results were formulated as an empirical correlation.     

Mixed Convection and Non-Gray Radiation in a Horizontal Rectangular Duct

Numerical studies for fluid flow and heat transfer in a horizontal rectangular duct are carried out. The flow is considered to be laminar, hydrodynamically and thermally developing. Heat transfer by both forced and natural convection is taken into account. The radiation from the gas is modeled with weighted sum of gray gases (WSGG) model. While considering non-gray radiation with WSGG, the fluid is considered to be a mixture of CO₂ and H₂O. Simulations are carried out with lower wall temperature than the inlet temperature of the gas. The effect of buoyancy and radiation on bulk mean temperature and Nusselt number are studied. The effects of temperature dependent properties are discussed. Comparative studies are carried out among forced convection, mixed convection, gray and non-gray gas radiation. It is found from the simulations that the assumption of gray gas can produce an error of ±10% over a non-gray model with WSGG for the cases studied.     

A novel case study for thermal radiation through a nanofluid as a semitransparent medium via discrete ordinates method to consider the absorption and scattering of nanoparticles along the radiation beams coupled with natural convection

Natural convection in a volumetric radiant enclosure filled by a nanofluid is studied numerically for the first time by using discrete ordinates (DO) method to consider the absorption and scattering coefficients of nanoparticles on the radiation beams through the nanofluid as a semitransparent medium. Present nanofluid is a mixture of Al₂O₃ nanoparticles suspended in water as the base fluid. The volume concentration percentages of nanoparticles are almost small to make a semitransparent medium which means the achieved results can be used in the flat plate solar collectors. Moreover the SIMPLE algorithm of finite volume method for Navier-Stokes continuity, momentum and energy equations are solved and coupled with DO to simulate the total radiation and natural convection in a shallow inclined rectangular 2-D enclosure. This shape of enclosure is chosen due to it might represent the usual configuration of a solar collector. The enclosure inner walls are the gray diffuse emitters and reflectors. The effects of various amounts of Rayleigh number and volume concentration at different values of wavelength are investigated. The positive effect of wave length on radiation heat flux and consequently total heat flux of radiation and natural convection is observed.     

Fouling in a Steam Cracker Convection Section Part 2: Coupled Tube Bank Simulation using an Improved Hybrid CFD-1D Model

Abstract

Performing an adequate fouling study for the heat exchangers in the convection section of a steam cracker requires reliable data on circumferential tube wall temperature profiles. A hybrid Computational Fluid Dynamics (CFD)-1D convection section model, developed to perform coupled flue gas/process gas side simulations of convection sections, is improved by the implementation of flue gas radiation modeling and extended to include typical tube banks. A complete naphtha cracker convection section is simulated with the improved hybrid CFD-1D model. All tubes show distinct maximum heat fluxes on the tube walls due to the high flue gas velocity. Based on the calculated circumferential heat flux profiles, the maximum heat flux value is calculated to be 1.8 times the average tube heat flux value. As computational costs associated with a hybrid CFD-1D simulation are high, a convective heat flux profile reconstruction scheme is developed. Using the scheme, circumferential heat flux profiles are reconstructed, based on the heat fluxes calculated when performing a fully 1D coupled convection section simulation. The heat flux reconstruction profile scheme enables fast retrieval of circumferential heat flux profiles and, thus, tube wall temperature profiles. Optimization and/or design of a steam cracker convection section becomes less computationally demanding.     

低温地板辐射采暖埋管结构尺寸设计实验研究

建立低温地板辐射采暖的传热模型,通过实测及理论计算,得出各表面之间的对流及辐射换热量,通过对换热表面的合理简化,得出地板辐射采暖表面温度与埋管层厚度、埋管间距的关系,为地板辐射采暖设计时确定埋管结构尺寸提供依据。     

Entropy generation in a hollow cylinder with temperature dependent thermal conductivity and internal heat generation with convective–radiative surface cooling

This article investigates entropy generation in an asymmetrically cooled hollow cylinder with temperature dependent thermal conductivity and internal heat generation. The inside surface of the cylinder is cooled by convection on its inside surface while the outside surface experiences simultaneous convective–radiative cooling. The thermal conductivity of the cylinder as well as the internal heat generation within the cylinder are linear functions of temperature, introducing two nonlinearities in the one-dimensional steady state heat conduction equation. A third nonlinearity arises due to radiative heat loss from the outside surface of the cylinder. The nonlinear system is solved analytically using the differential transformation method (DTM) to obtain the temperature distribution which is then used to compute local and total entropy generation rates in the cylinder. The accuracy of DTM is verified by comparing its predictions with the analytical solution for the case of constant thermal conductivity and constant internal heat generation. The local and total entropy generations depend on six dimensionless parameters: heat generation parameter Q, thermal conductivity parameter β, conduction–convection parameters Nc₁ and Nc₂, conduction–radiation parameter Nr, convection sink temperature δ and radiation sink temperature η.     

Heat transfer analysis of solar-driven high-temperature thermochemical reactor using NiFe-Aluminate RPCs

Converting solar energy efficiently into hydrogen is a promising way for renewable fuels technology. However, high-temperature heat transfer enhancement of solar thermochemical process is still a pertinent challenge for solar energy conversion into fuels. In this paper, high-temperature heat transfer enhancement accounting for radiation, conduction, and convection heat transfer in porous-medium reactor filled with application in hydrogen generation has been investigated. NiFe-Aluminate porous media is synthesized and used as solar radiant absorber and redox material. Experiments combined with numerical models are performed for analyzing thermal characteristics and chemical changes in solar receiver. The reacting medium is most heated by radiation heat transfer and higher temperature distribution is observed in the region exposed to high radiation heat flux. Heat distribution, O₂ and H₂ yield in the reacting medium are facilitated by convective reactive gas moving through the medium's pores. The temperature gradient caused by thermal transition at fluid-solid interface could be more decreased as much as the reaction chamber can store the transferred high-temperature heat flux. However, thermal losses due to radiation flux lost at the quartz glass are obviously inevitable.     

A unified heat transfer model in a pressurized volumetric solar receivers

In the present work, we developed an overall mathematical model adequately describing the main heat transfer processes in a pressurized volumetric receiver. The key components, a windowed cavity, incorporating with the irradiated surface of the absorber, were theoretically modeled as a closed diffuse-gray surfaces system. Accordingly, a boundary condition for the absorber concerning its porous structure surface was developed using net radiation method (NRM) under local thermal non-equilibrium (LTNE) condition. The same method is also applied to the back cavity. Then a modified P1 approximation with collimated irradiation was introduced to incorporate the radiation transfer penetrating in the absorber. The major characteristic of the heat transfer behavior combining radiation, thermal conduction, and convection in the windowed cavity, absorber and the back cavity, are detailedly presented. Also, the key design parameters, such as those relating to pore structure (φ and d_p), the volumetric heat transfer coefficient h_v, the emissivity ε for window and absorber, and their thickness L_a and L_g were systematically analyzed. Optimization design can be carried out for both of the solar thermal system and the receiver itself in the future work based on our model.     

Numerical assessment of heat transfer coefficient for preterm infant nursed under a radiant warmer

Body heat loss management is the primary concern with respect to a newborn, as excess heat loss or gain leads to hypo- or hyperthermia, respectively. The aim of this article is to numerically investigate the convection and radiation heat transfer coefficient of a newborn nursed under a radiant warmer. The preterm neonate manikin has five body segments (head, trunk, leg, arms, and back) placed in a relaxed spine position. In the present study, numerical simulations are carried out for body temperature ranging between 32.5°C and 40.1°C. Ambient air temperature is chosen from 22.77°C to 30°C as preferred room temperature in the analysis. Airflow and heat transfer under the radiant warmer are analyzed in two operational modes, that is, power-off and power-on modes. In the power-on mode, the convective heat transfer coefficient varies between 1.45 and 4.06 W/m² K, whereas the radiative heat transfer coefficient varies between 0.08 and 3.28 W/m² K under various operating conditions. The results obtained are numerically validated and found to be in fair agreement with the experimental results available in the open literature.     

Experimental study of forced and free convective heat transfer in the thermal entry region of horizontal concentric annuli

Forced and free convective heat transfer for thermally developing and thermally fully developed laminar air flow inside horizontal concentric annuli in the thermal entrance length has been experimentally investigated. The experimental setup consists of a stainless steel annulus having a radius ratio of 2 and an inner tube with a heated length of 900 mm subjected to a constant wall heat flux boundary condition and an adiabatic outer annulus. The investigation covers Reynolds number range from 200 to 1000, the Grashof number was ranged from 6.2 × 10⁵ to 1.2 × 10⁷. The entrance sections used were long tube with length of 2520 mm (L/D_h = 63) and short tube with length of 504 mm (L/D_h = 12.6). The surface temperature distribution along the inner tube surface, and the local Nusselt number distribution versus dimensionless axial distance Z_t were presented and discussed. It is inferred that the free convection effects tended to decrease the heat transfer at low Re number while to increase the heat transfer for high Re number. This investigation reveals that the Nusselt number values were considerably greater than the corresponding values for fully developed combined convection over a significant portion of the annulus. The average heat transfer results were correlated in terms of the relevant dimensionless variables with an empirical correlation. The local Nusselt number results were compared with available literature and show similar trend and satisfactory agreement.     

Numerical Investigation of Conjugate Heat Transfer to Supercritical CO2 in a Vertical Tube-in-Tube Heat Exchanger

Conjugate heat transfer to supercritical CO₂ in a vertical tube-in-tube heat exchanger was numerically investigated. The results demonstrate that most models considered are able to reproduce the heat transfer processes qualitatively, and the Abe, Kondoh, and Nagano model shows optimal agreement with the experimental data. The influences of hot fluid mass flux and temperature of the shell side, supercritical fluid mass flux of the tube side, flow direction, and pipe diameter on conjugate heat transfer were investigated based on velocity and turbulence fields. It is concluded that hot fluid mass flux and temperature of the shell side significantly affect heat transfer of the tube side. Mixed convection is the main heat transfer mechanism for the supercritical CO₂ conjugate heat transfer process when the inner diameter of the tube is greater than 1 mm. In addition, density variation is highly significant for heat transfer of supercritical CO₂ while high viscosity hinders the distortion of the flow field and reduces deterioration in heat transfer.     

An analytical study of pulsating laminar heat convection in a circular tube with constant heat flux

Pulsating laminar convection heat transfer in a circular tube with constant wall heat flux is investigated analytically. The results show that both the temperature profile and the Nusselt number fluctuate periodically about the solution for steady laminar convection, with the fluctuation amplitude depending on the dimensionless pulsation frequency, ω*, the amplitude, γ, and the Prandtl number, Pr. It is also shown that pulsation has no effect on the time-average Nusselt numbers for pulsating convection heat transfer in a circular tube with constant wall heat flux.     

Experimental evaluations of the building shell radiant exchange in clear sky conditions

The results of an experimental investigation on the exchange of radiant energy in the infrared band (λ = 4–100 μm) between the walls of a building and the surrounding environment, constituted in part by the sky and in part by the ground, are presented in this paper. The measures were obtained on a purpose built test module for studies and research on building energy. Through measurements of infrared radiation on a South facing vertical wall and on a horizontal roof, and of the relative surface temperatures, the radiant field between these surfaces and the outdoor environment was resolved in conditions of clear hourly diurnal and nocturnal sky. The investigation allowed for the determination of the hourly values of the radiative heat transfer coefficients between the vertical wall and the sky, between the vertical wall and the ground and between the horizontal roof and the sky. Furthermore, EN ISO 13790:2008, which is used for the evaluation of energy requirements of building air-conditioning, was considered and the various contributions used in order to evaluate the radiative exchange with the experimentally obtained values were compared.