Numerical investigation of supercritical LNG convective heat transfer in a horizontal serpentine tube

The submerged combustion vaporizer (SCV) is indispensable general equipment for liquefied natural gas (LNG) receiving terminals. In this paper, numerical simulation was conducted to get insight into the flow and heat transfer characteristics of supercritical LNG on the tube-side of SCV. The SST model with enhanced wall treatment method was utilized to handle the coupled wall-to-LNG heat transfer. The thermal–physical properties of LNG under supercritical pressure were used for this study. After the validation of model and method, the effects of mass flux, outer wall temperature and inlet pressure on the heat transfer behaviors were discussed in detail. Then the non-uniformity heat transfer mechanism of supercritical LNG and effect of natural convection due to buoyancy change in the tube was discussed based on the numerical results. Moreover, different flow and heat transfer characteristics inside the bend tube sections were also analyzed. The obtained numerical results showed that the local surface heat transfer coefficient attained its peak value when the bulk LNG temperature approached the so-called pseudo-critical temperature. Higher mass flux could eliminate the heat transfer deteriorations due to the increase of turbulent diffusion. An increase of outer wall temperature had a significant influence on diminishing heat transfer ability of LNG. The maximum surface heat transfer coefficient strongly depended on inlet pressure. Bend tube sections could enhance the heat transfer due to secondary flow phenomenon. Furthermore, based on the current simulation results, a new dimensionless, semi-theoretical empirical correlation was developed for supercritical LNG convective heat transfer in a horizontal serpentine tube. The paper provided the mechanism of heat transfer for the design of high-efficiency SCV.     

An integral method of calculation of stabilized heat transfer in tubes in single-phase near-critical region

The modified method of calculation of “stabilized” heat transfer in tubes, which was developed previously and tested with gases of varying properties, is adapted to the conditions of single-phase near-critical region. Calculations are performed for water at a pressure from 22.5 to 29.4 MPa in the enthalpy range from 500 to 3500 kJ/kg under conditions of turbulent flow in heated tubes 4 to 16 mm in diameter and with heat loads from 0.2 to 1.2 kJ/kg. The calculation data are compared with known empirical formulas of different authors and used for refining the practical methods of calculation of heat transfer and drag under conditions of normal heat transfer. The question is discussed of the consequences of changing to a new standard of properties of water in the supercritical region of parameters from the standpoint of accuracy of thermohydraulic calculations.     

New correlation to predict the heat transfer coefficient during in-tube cooling of turbulent supercritical CO2

The Nusselt number variations of supercritical carbon dioxide during in-tube cooling are presented and discussed. Using data presented in this paper as well as prior publications, a new correlation to predict the heat transfer coefficient of supercritical carbon dioxide during in-tube cooling has been developed. The new correlation is presented in this paper. It is based on mean Nusselt numbers that are calculated using the thermophysical properties at the wall and the bulk temperatures, respectively. It is seen that the majority of the numerical and experimental values are within ±20% of the values predicted by the new correlation.     

Subcooled liquid boiling heat transfer under conditions of stepwise heat-load release

The results of experimental investigations of the effect of liquid subcooling below the saturation temperature on boiling heat transfer under conditions of an increase in a heating load are presented. Water and TS-1 kerosene were used as heat carriers. The obtained experimental data on subcooled liquid boiling heat transfer has been generalized by an empirical relation. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 80, No. 1, pp. 136–139, January–February, 2007.     

Numerical modeling of heat exchange and turbulent flow of fluid within tubes at supercritical pressure

Modes of normal and degraded (with peaks of wall temperature) heat transfer are computed for the turbulent flow of carbon dioxide within a circular tube at supercritical pressure. Computation is based on a set of motion, continuity, and energy equations written under the approximation of a narrow channel. The turbulence model uses the Prandtl formula for the turbulent viscosity. The relationship for the travel length takes into account the effect of variation in the fluid properties and thermal acceleration through the tube section. Computation results for variation in the wall temperature along the tube fit the experimental data. An explanation is given for causes of the appearance of the peak on the wall temperature distribution along the tube in the area, where the fluid temperature is close to the pseudocritical temperature.     

In-tube cooling heat transfer of supercritical carbon dioxide. Part 2. Comparison of numerical calculation with different turbulence models

Heat transfer of supercritical carbon dioxide cooled in circular tubes was investigated experimentally. The effect of mass flux, pressure, and heat flux on the heat transfer coefficient and pressure drop was measured for four horizontal cooling tubes with different inner diameters ranging from 1 to 6 mm. The radial distribution of the thermophysical properties (i.e. specific heat, density, thermal conductivity and viscosity) in the tube cross-section was critical for interpreting the experimental results. A modified Gnielinski equation by selecting the reference temperature properly was then developed to predict the heat transfer coefficient of supercritical carbon dioxide under cooling conditions. This proposed correlation was accurate to within 20% of the experimental data.     

Influence of thermophysical properties on pool boiling heat transfer of refrigerants

The correct prediction of the heat transfer performance of the boiling liquid within the evaporator of a refrigeration unit is one of the essential features for the successful operation of the whole unit. A theoretically consistent calculation method for the heat transfer coefficient α in nucleate boiling, which should be based on the physical phenomena connected with vapour bubbles growing, departing and sliding on the wall and with the interactions of bubbles and of neighbouring nucleation sites within the microstructure of the heating surface, does not yet exist, despite the increasing number of papers on the subject in the recent past. Instead, the predictive methods for α available at present are empirical or semiempirical, especially for heat transfer conditions relevant in practice. Many of these correlations have been established in the form of power laws in which the relative influences of the main groups of variables on α are treated by separate factors. One of these may stand for the influence of the thermophysical properties of the boiling liquid or these properties will be included in several of the factors.New experimental results are presented for pool boiling heat transfer from a single horizontal copper tube (8 mm OD) to HFC-refrigerants (R32, 125, 134a, 143a, 152a, 227ea) and hydrocarbons (propane, i-butane). The results are compared to experimental data from the literature, and methods are discussed, how to incorporate the data in semiempirical correlations to describe the influence of the thermophysical properties of the fluids on the heat transfer performance.     

Experimental investigation on heat transfer characteristics of supercritical CO2 cooled in horizontal helically coiled tube

An experimental investigation on the heat transfer characteristics of supercritical CO₂ during gas cooling process in a helically coiled tube is conducted. The experimental data are obtained over a mass flux range of 79.6–238.7 kg m⁻² s⁻¹, an inlet pressure range of 7.5–9.0 MPa and a mean bulk temperature of 23.0–53.0 °C. The effects of mass flux, bulk temperature and pressure on the heat transfer coefficient for helically coiled tubes are investigated. A comparative analysis of the gravitational buoyancy and the heat transfer coefficient is carried out between helically coiled tubes and straight tubes. A new heat transfer correlation of the supercritical CO₂ in the horizontal helically coiled tube is proposed based on the experimental data. The maximum error between the predicted results of the new correlation and the experimental data is 20%.     

The nature of wall temperature distributions in regions of deteriorated heat transfer

This note concerns the difficulty of fitting heat transfer correlations to data in deteriorated forced convective flows of supercritical pressure fluids. Experimental data suggests that the heat transfer coefficient depends inversely upon wall to bulk temperature difference, causing the wall temperature to become very sensitive to secondary features such as axial conduction in the test section wall. Under certain conditions the wall temperature level cannot be uniquely determined by a correlation. It is suggested that by noting the transient response of wall temperature to step changes in heat flux the influence of axial conduction may be deduced.     

Characteristics of self-excited thermoacoustic oscillations in heat transfer to n-heptane

The speed of sound and wavelength are determined experimentally for thermoacoustic oscillations self-excited in connection with heat transfer to a flow of n-heptane under supercritical pressure conditions.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 33, No. 1, pp. 21–25, July, 1977.     

Special features of heat transfer under conditions of heat protection of high-temperature firing walls of components of modern power-generating units by way of tangential injection

In order to more precisely define the characteristics of heat transfer under conditions of protection of firing wall by means of tangential injection in the case of its high temperature (in particular, higher-thanadiabatic temperature) and to assess the effect of degree of turbulence of the incoming gas flow on heat transfer, a numerical investigation is performed under conditions of parameters typical of combustors of gas-turbine plants (GTP) with high parameters of the working medium. In so doing, the heat flux distribution, the profiles of turbulence intensity, the distribution of turbulent viscosity in the injection zone region under study, and other characteristics are determined. The low-Reynolds k-ε model with wall functions and a new model of turbulent viscosity without wall functions are employed. It is found that a maximum of turbulent viscosity takes place behind the exit section of the injection slit with a shift to the main flow under conditions of tangential injection on an isothermal surface with a temperature much in excess of injection temperature (in a more general case, T _w > T _ad). This causes impairment of heat protection by injection, i.e., an increase in heat fluxes in the computational domain compared to heat fluxes calculated using integral methods.     

超临界二氧化碳管内流动及换热特性分析研究

通过对超临界二氧化碳管内流动及换热特性研究现状和分析方法介绍,列出常用的超临界二氧化碳在不同条件下的传热和压降关联式,进一步说明自然工质二氧化碳的跨临界循环特点和所具有的独特的热物理性质,指明超临界二氧化碳的利用和新型换热设备的研发方向。     

HFE-7100 pool boiling heat transfer and critical heat flux in inclined narrow spaces

Experiments were performed to examine the pool boiling heat transfer and critical heat flux on a smooth copper circular surface, confined by a face-to-face parallel unheated surface, by changing both the orientation and the gap between the surfaces. Pool boiling data at atmospheric pressure were obtained for saturated HFE-7100. The gaps between the boiling surface and adiabatic one were 0.5, 1.0, 2.0, 3.5, 5.0, 10.0 and 20.0 mm. For each configuration, boiling curves were obtained up to the thermal crisis. The surface orientations investigated were 0° (horizontal upward surface), 45°, 90° and 135°. It was observed that the heat transfer coefficient improves at low wall superheat when the distance between heated and unheated surfaces decreases. However, at high wall superheat, a drastic reduction in heat transfer as well as CHF appears for confined boiling. For a fixed channel width, the CHF value strongly depends on the channel orientation, assuming a maximum value for near-vertical channels with up-facing heated surface. CHF data were compared with various literature correlations; a new empirical correlation that takes into account channel gap effects is proposed for horizontal confined surfaces.     

Heat transfer to turbulent stream in pipes under supercritical pressures

Experimental results are presented on heat transfer to several liquid hydrocarbons under supercritical pressures and pseudoboiling conditions. An empirical relation is proposed which generalizes these results.     

Nature of heat exchange and hydraulic resistance under conditions of forced movement of a liquid at supercritical pressure

The results are presented of an experimental study of hydraulic resistance and high-frequency pulsations in pressure arising during the process of heat exchange at supercritical pressure of n-heptane as a function of wall temperature, flow rate, and length of the working channel.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 21, No. 2, pp. 219–224, August, 1971.     

Heat exchange during the forced flow of hydrocarbon fuels at supercritical pressures in heated tubes

Heat transfer coefficients have been measured during the turbulent flow of hydrocarbon fuels at supercritical pressures under conditions in which no deposits occur. It is shown that a regime of impaired heat exchange can occur when the stream and wall temperature values are lower than the pseudocritical temperature of the fuel. The boundaries of this regime are established and empirical relationships are proposed for calculating the impaired and normal heat exchange.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 60, No. 1, pp. 46–50, January, 1991.     

Analysis of heat flow and “channelling” in a scraped-surface heat exchanger

Scraped-surface heat exchangers (SSHEs) are widely used in industries that manufacture and thermally process fluids; in particular, the food industry makes great use of such devices. Current understanding of the heat flow and fluid dynamics in SSHEs is predominantly based on empirical evidence. In this study a theoretical approach (based on asymptotic analysis) is presented for analysing both the flow and heat transfer in an idealised SSHE (a cylindrical annulus) for Newtonian fluids. The theory allows the effects of scraping-blade configuration, pumping rates, annular shear velocity and material properties all to be accounted for. The analysis relies on asymptotic simplifications that result from the large Péclet numbers and small geometrical aspect ratios that are commonly encountered in industrial SSHEs. The resulting models greatly reduce the computational effort required to simulate the steady-state behaviour of SSHEs and give results that compare favourably with full numerical simulations. The analysis also leads to what appears to be the first theoretical study on the undesirable phenomenon of “channelling”, where fluid passes through the device in an essentially unheated or uncooled state. A parametric study is also undertaken to investigate the general circumstances under which channelling may occur.     

超临界二氧化碳流动和换热研究进展

综述了国际上对超临界二氧化碳管内换热和压降特征的研究。提供了多种公开发表的超临界流体在冷却工况下的换热关联式及单相压降关联式,将实验关联式的计算结果与文献中的实验数据进行对照,在此基础上对关联式的准确性进行了讨论。同时指出了现有研究内容的不足。     

Failure analysis of 101-C ammonia plant heat exchanger

Long-term overheating is a major cause of failures in boiler tubes of different designs and operation conditions. These failures are usually related to the formation and growth of internal oxide scales or deposits at areas of high heat flux. Deposits might have different sources; poor water treatment and general corrosion in the system are two common types of these sources. Thickening of the deposit layer hinders heat transfer and local spots of high metal temperature are created in the tube wall. With increasing metal temperature, creep becomes much faster leading to tube failure under internal pressure. In this paper, a case history involving this type of failure is presented. Using finite element modeling the failure are analyzed and some methods to prevent this type of failure are recommended.     

NH3 in-tube condensation heat transfer and pressure drop in a smooth tube

This paper presents an overview of the issues and new results for in-tube condensation of ammonia in horizontal round tubes. A new empirical correlation is presented based on measured NH₃ in-tube condensation heat transfer and pressure drop by Komandiwirya et al. [Komandiwirya, H.B., Hrnjak, P.S., Newell, T.A., 2005. An experimental investigation of pressure drop and heat transfer in an in-tube condensation system of ammonia with and without miscible oil in smooth and enhanced tubes. ACRC CR-54, University of Illinois at Urbana-Champaign] in an 8.1 mm aluminum tube at a saturation temperature of 35 °C, and for a mass flux range of 20–270 kg m⁻² s⁻¹. Most correlations overpredict these measured NH₃ heat transfer coefficients, up to 300%. The reasons are attributed to difference in thermophysical properties of ammonia compared to other refrigerants used in generation and validation of the correlations. Based on the conventional correlations, thermophysical properties of ammonia, and measured heat transfer coefficients, a new correlation was developed which can predict most of the measured values within ±20%. Measured NH₃ pressure drop is shown and discussed. Two separated flow models are shown to predict the pressure drop relatively well at pressure drop higher than 1 kPa m⁻¹, while a homogeneous model yields acceptable values at pressure drop less than 1 kPa m⁻¹. The pressure drop mechanism and prediction accuracy are explained though the use of flow patterns.