A NUMERICAL STUDY ON THE FLOW AND HEAT TRANSFER FOR THE INSIDE OF A NEW DIVERSION-TYPE LNG HEATING DEVICE

Proceedings of CHT-15 ICHMT International Symposium on Advances in Computational Heat Transfer May 25-29, 2015, Rutgers University, Piscataway, NJ, USA CHT-15-135 A NUMERICAL STUDY ON THE FLOW AND HEAT TRANSFER FOR THE INSIDE OF A NEW DIVERSION-TYPE LNG HEATING DEVICE Yun Guo *,**,, Zhixiong Guo **, Hongming Li * and Zhiguan Zhou * * College of Mechanical Engineering, Shanghai University of Engineering Science, China ** Department of Mechanical and Aerospace Engineering, Rutgers, The State University of New Jersey, USA Correspondence author. Email: graceguo1977@126.com ABSTRACT LNG heating and gasification device is an indispensable piece of equipment in gas application systems. The present paper characterizes the inside flow and heat transfer of a new type LNG heating device with various guide plate structures. The natural convection model for both heating and cooling surfaces in the heat-exchanging cylinder is built. The finite volume method with unstructured body-fitted grids is employed. Analyses and comparisons of the flow conditions and temperature distributions with a single side completely blocking guide plate, or a single side partly blocking arc guide plate, or two side guide plates between the heating and cooling surfaces are carried out. The numerical simulation shows that using the new type of guide plate structures can form an overall smooth flow field effectively. Use of either a single side guide plate or two side guide plates can make the thermal boundary layer close to the wall thinner, and help the heat-transfer medium form an organized flow pattern that enhances the heat transfer. Such a diversion-type heat transfer mode is especially suitable for high viscosity heat-transfer media such as ethylene glycol. KEYWORDS LNG heating; natural convection; guide plates; thermal efficiency; ethylene glycol INTRODUCTION Since liquefied natural gas (LNG) must be heated to vaporization before being used, the vaporization heating equipment is indispensable in the relatively independent areas where the natural gas supplying net is not connected, and in the regions with large load variation range such as in industries that require frequently quick gas supply or stop [Cheng 2010]. Since LNG is flammable and explosive, the standards [1997] demand that it be heated by an intermediate heating medium such as water/ethylene glycol solution, rather than by flame or flue gas. A LNG heating device adopts a solid assembled structure, as shown in Fig. 1. Generally, the heating surfaces (fire tube and flue tube bundle) and the cooling surfaces (convective tube bundle) are symmetrically arranged by the central axis of the circular cross-section placed inside the large cylinder. The heating surface is placed below the horizontal axis and the cooling surface is placed above the horizontal axis. The cylinder is filled with a heat-transfer medium, which can be ethylene glycol [Guo 2012]. Heat from the fuel combustion is firstly transferred to the ethylene glycol through the fire tube wall and flue tube bundle wall. At the same time, the ethylene glycol transfers most of

the heat to the LNG in the convective tube bundle. The ethylene glycol in the cylinder flows due to the buoyancy, which is generated by the uneven density caused by the temperature difference. 1: Burner 2: Fire tube 3: Flue tube bundle 4: Inlet of the convective tube bundle 5: Outlet of the convective tube bundle 6: Chimney Fig. 1. Scheme of a LNG heating device Natural convective heat transfer occurs between the ethylene glycol and the heating/cooling surface. By testing the temperature distribution of ethylene glycol in the cylinder of a LNG heating device with conventional arrangement of heat-exchange surfaces, Guo et al. [2009] found the followings. First, the temperature of the ethylene glycol close to the wall of the fire tube and flue-tube bundle was higher and the ethylene glycol fluid in this lower region of the cylinder forms an uprising flow; while the temperature of the ethylene glycol close to the convective tube bundle was lower and the ethylene glycol in this upper region forms a descending flow. Because there is no a separate flow channel, these two counter-opposed flows impact each other. Additionally, there exist temperature differences between the fire tube and flue tube bundle and between the multi-return passes of the convective tube bundle. All these can cause chaos in the ethylene glycol flow and result in a poor flow pattern and uneven heating. Second, the driving force for the natural convection is small due to the small temperature difference between the up and low parts of the flow field. The flow formed could hardly be effective such that the convective heat transfer is very weak between the heating surface and cooling surface. In some cases, the temperature difference between the cold and hot fluids is negligible, such that the flow field in the cylinder may even be considered in a static state and the heat transfer mode would be close to heat conduction. It results in a very low heating efficiency for the LNG heating device. In order to increase the thermal efficiency by overcoming the disadvantages stated above, a new structure with clapboard was proposed to optimize the heat transfer in the cylinder with use of high-viscidity ethylene glycol. Depending on the different usage conditions and the technical requirements, the guide plates can be designed with different shapes and arrangements to fit different applications. The shape of guide plate can be designed as flat or arc and the guide plate arrangement can be set as one side or two sides. In this study, the effects of guide plate on fluid flow and temperature distribution will be investigated numerically. NUMERICAL SIMULATION Along with the length direction of the cylinder, the flow field in most the middle section is stable, except at the two ends, due to the bundle entrance and bend effects. To simplify the problem, the end effects are ignored in this study. The two-dimensional circular cross-section of the cylinder is shown

in Fig. 2. The physical properties during calculations were assumed to be constant, except for the density. The density is assumed to be linearly proportional to the temperature, because the medium temperature difference is small. The fluid flow is considered as a steady-state laminar flow, so that the viscous dissipation is negligible. The outer wall is treated as an adiabatic boundary and the heat losses there are ignored. Fig. 2. The cross-sectional view of the heating device circular structure Based on the Boussinesq assumption under the steady-state laminar flow condition, the 2-D flow and heat transfer governing equations are listed below [Patankar 1984 and Wassim et al. 2002]. Continuity equation: u v + = 0 (1) Momentum equations: u u p u u ρ ( u + v ) = + µ ( + ) (2) v v p v v ρ( u + v ) = + µ ( + ) + ρgβ ( T T ) (3) Energy equation: T T k T T ρ ( u + v ) = ( + ) (4) c Where, u, v are the velocity components on x, y directions respectively, m/s; T is the temperature, ºC; µ is the fluid dynamic viscosity coefficient, kg/(m s); T is the fluid reference temperature, ºC; β is the fluid volume expansion coefficient, K -1 ; ρ is the fluid density, kg/m 3 ; k is the fluid thermal conductivity, w/(m ºC); and c p is the fluid specific heat, kj/(kg ºC). Equations (1)-(4) can be converted to non-dimensional forms, using the following non-dimensional parameters: x y ud vd p k X =, Y =, U =, V =, P =, Θ = ( t t ) (5) 2 D D a a ρ ( a/ D) qd p

The non-dimensional continuity, momentum and energy equations are: U V + = 0 (6) U U P U U U + V = + Pr( + ) (7) V V P V V k U + V = + Pr( + 2 ) + Ra 2 D P r ( t t ) qd (8) Θ Θ Θ Θ U + V = a( + ) + S (9) The Rayleigh number and the Prandtl number are: Ra D 4 gβqd = GrD Pr =, kυa ν P r = (10) a Where, a is the thermal diffusivity, q is the heat flux, W/m 2. k a =,m 2 /s; υ is the kinematic viscosity, ρ c p µ υ =,m 2 /s; ρ The boundary conditions, used to solve the eqs. (6)-(9) are: Θ on outer wall of cylinder U =V =0, w = 0 n (11) Θ on heating and cooling surfaces U =V =0, w = 1 n Because of the complex layout of the heat-exchange surfaces, how to mesh the grid effectively is the most difficult issue in the simulation [Xie 2005 and Zhang 2010]. In this paper, due to the limitation of the large cylinder body structure, the body-fitted solution is introduced in the Cartesian coordinate system and the unstructured grids are adopted to mesh the irregular surfaces and surrounding areas. Not only the generated grid system is simple and intuitive, but also the conversions of node coordinates and the governing equations in the physical and computational domain are avoided [Boz et al. 2014 and Berger et al. 1989]. Fig. 3 shows the computation mesh on the two-dimensional circular cross-section of the cylinder, mesh cells are 130935. To ensure that the obtained results were mesh-independent, the number of elements in the computational domain was increased by 25% and 50%. The maximum relative error of the temperature on the horizontal axis was computed, and the difference between the models was less than 2.3%. Therefore, the original model was adopted for the numerical simulations. Based on the finite volume method, the commercial software package Fluent 6.3 is used for the simulation of the fluid flow and heat transfer in the cylinder. The second-order windward difference scheme is adopted for the convection diffusion terms and the pressure and velocity are coupled by a simple algorithm. In order to increase the calculation efficiency, the buoyancy driven flow strategy is also adapted, and the relative error is less than 4%, as shown by Guo et al. [2009].

Fig. 3. Grid system The flow conditions and temperature distributions with guide plate are simulated by using the above established natural convection model. The inside guide plate is regarded as adiabatic. In accordance with the actual operating conditions of an LNG heating device, the calculation conditions are specified as follows: the medium is ethylene glycol, 80.8 kw and 2.48 m 2 are for the heat flux and area of the heating surface of the fire tube, 21.0 kw and 8.56 m 2 are for the flue tube bundle; 46.5 kw and 4.93 m 2 are for the heat flux and area of the cooling surface of the first and second passes of the convective tube bundle, 47.3 kw and 4.93 m 2 are for the third and fourth passes of the convective tube bundle. RESULTS AND DISCUSSION The temperature and flow fields for the single-side guide plate are shown in Figs. 4 (a) and (b), respectively. A guide plate with length 350 mm and thickness 6 mm is placed between the fire tube and the convective tube bundle. One side of the guide plate is tangent to the right side of the fire tube and the other side is connected with the convective tube at the left bottom side of the first pass of the convective tube bundle. (a) Isothermal contour (K) (b) Flow vectogram (m/s) Fig. 4. Temperature and flow fields of the heating device with a single-side flat guide plate (Medium: ethylene glycol;working temperature: 380 K) As seen from the temperature distribution in Fig. 3(a), a long narrow high temperature zone clings to the left side of the guide plate, while a high temperature horn-shaped zone appears on both sides of the flue tube bundle, which points to both sides of the channel of the above cooling surface. An obvious descending flow trend is formed around the first and second passes of the convective tube bundle and the right side of the guide plate appears a lower temperature down-flow zone. From the flow vectogram in Fig. 3(b), it is seen that the guide plate blocks the channel between the fire tube

and the first pass of the convective tube bundle. As a result, the ethylene glycol releases heat while flows through the first and second passes of the convective tube bundle along the guide plate after absorbing heat from the fire tube and flue tube bundle below, and then flows back to the fire tube along the other side of the guide plate. A big circulation flow field is formed. Because of no guide plate at the left side, the ethylene glycol around the flue tube bundle lacks effective lead, which causes a local hedging with the descending flow around the above convective tube bundle. A guide plate can be installed for not only totally blocking, but also partly blocking. In order to obtain a smoother flow field, an arc guide plate could be a better choice. A partly blocking arc guide plate is placed between the fire tube and the convective tube bundle. The guide plate is tangent to the right side of fire tube and extends to the top left. The channel between the fire tube and the first pass of the convective tube bundle is partly blocked, and the plate is about 320 mm long and 6 mm thick. Figs. 5 (a) and (b) show the temperature and flow fields, respectively, for the device with this single-side partly blocking arc guide plate. (a) Isothermal contour (K) (b) Flow vectogram (m/s) Fig. 5. Temperature and flow fields of the heating device with a single-side arc guide plate (Medium: ethylene glycol;working temperature: 380 K) Fig. 5(a) shows that a narrow arc high temperature band appears on the top of the fire tube and a high temperature horn-shaped zone also appears on both sides of the flue tube bundle, which points to both sides of the channel of the cooling surface above. An obvious descending flow trend is formed in the surrounding of the first and second passes of the convective tube bundle and the right side of the guide plate presents a lower temperature down-stream zone. In Fig. 5(b), due to the joint effect of thermal pressure and partly blocking arc guide plate, the ethylene glycol around the fire tube flows upward along the curved guide plate, and then mixes with the ethylene glycol around the flue tube bundle; flows through the multi-return of the convective tube bundle in turn. After the heat release process, the ethylene glycol will drop reflux back to near the fire tube along the other side of the arc guide plate. A clockwise and smoother large flow circulation is formed, due to the better pertinence between the arc guide plate and streamline of the heat flow. Therefore, it enhances the heat transfer. In some cases, if the thermal pressure between the upper and lower parts of the flow field is small and the efficiency with a single plate is poor, the guide plates could be set at the both sides of the circular section between the heating and cooling surfaces respectively to form a smooth and uniform heat flow field with double flow circulations. Figs. 6 (a) and (b) show the temperature contours and velocity vector maps respectively, by installing double guide plates. The guide plates are respectively set at the place between the right side of the fire tube and the left side of the first pass of the convective tube bundle and the place between the left side of the flue tube bundle and the right side of the fourth pass of the convective tube bundle. The

plates are about 350 mm long and 6 mm thick. As seen from the temperature contour in Fig. 6(a), the narrow high temperature zone on the top of the fire tube is close to the right side of the guide plate, while the high temperature zone above the flue tube bundle is close to the left side of the guide plate. A down welling trend is formed at the bottom right of the first and second passes of the convective tube bundle and a lower temperature down-stream region is appeared. Moreover, a down welling trend is also formed at the bottom left of the third and fourth passes of the convective tube bundle, and a lower temperature down-stream zone is also appeared. As seen from the velocity vector map in Fig. 6(b), with the guiding of double-side guide plates, the ethylene glycol around the fire tube goes upward and flows through the convective tube bundle in the right region, then comes back to near the fire tube. And the ethylene glycol around the flue tube goes upward and flows through the convective tube bundle in the left region, then comes back to near the flue tube bundle. Hence, a double-side circulation flow field of circumfluence from center to two-side is formed on the circular cross-section in the cylinder. (a) Isothermal contour (K) (b) Flow vectogram (m/s) Fig. 6. Temperature and flow fields of the heating device with double side guide plates (Medium: ethylene glycol;working temperature: 380 K) No matter what kind of arrangement is adapted, the guide plates separate the flow region between the heating surfaces and cooling surfaces reasonably. Obviously, the guide plates make the thermal boundary layer close to the wall thinner, guide the heat-transfer medium to form an overall organized smooth flow, and the flow dead angles are eliminated. Thereby, the thermal efficiency is improved. Such a diversion-type heat transfer mode is especially suitable for the heat-transfer medium whose viscosity is great such as the ethylene glycol. So, in terms of the special heat transfer structure of the heating device and the properties of the heat-transfer medium, installing guide plates to enhance heat transfer is superior to other enhancement means. CONCLUSIONS (1) To overcome the weakness that an effective heat flow field cannot be formed inside the conventional LNG heating device, use of various guide plates installed between the heating and cooling surfaces to improve the overall heat transfer efficiency is considered. The simulated results in this study demonstrated the feasibility and advantages of such a heat transfer structure within a LNG heating device. (2) Installing guide plates along the axial direction of the large cylinder is a simple and effective heat transfer enhancement method, especially for high viscosity heating medium such as ethylene glycol. An effective and smooth flow field can be formed in the LNG heating device. (3) The diversion-type LNG heating device has the merits of simple structure, wide application range, and quick start, etc. The cost for installation of guide plates is low and can be recouped in a short

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