INTENSIFIED HEAT TRANSFER TECHNOLOGIES FOR ENHANCED HEAT RECOVERY Project meeting July 8, 2011 Veszprem, Hungary WP1 - Enhancing understanding of heat exchange under fouling Professor Barry Crittenden and Dr Mengyan Yang University of Bath

TYPICAL GROWTH IN HEAT EXCHANGER FOULING RESISTANCE Thermal resistance Date

PROJECT UPDATE (WP1) 1. To gain in-depth understanding of fouling mechanisms and kinetics of fouling through experiments Progress: Crude oil fouling tests using stirred cell with modified probe CaSO 4 crystallization fouling tests Overall Objective Enhancing understanding of heat exchange under fouling Deliverable: Technical review of fouling and its impact on heat transfer – delivered on April 2011 Specific Objectives 2. To develop an advanced CFD tool to improve the heat exchanger performance by adjusting both operating conditions and equipment geometry. Progress: CFD simulation of heat and momentum transfer in tube fitted with hiTRAN inserts and in the stirred cell with modified probe

Induction period: - Surface Coverage, θ: The active species stick onto the surface and gradually cover the surface (to θ = 1). Fouling period: The fouling layer starts to grow immediately on the covered surface, and the over all fouling growth rate is proportional to θ. R’ can be any established fouling rate expressions dθ/dt is proportional to the percentage of uncovered surface, (1 – θ), and particles stuck on the surface act as seeds, growing in a micro growth manner, as such, dθ/dt is also a 1 st order to θ, Hence: Growth rate = k 1 θ(1-θ) Further development - Adopting the concept of removal from the surface as in adsorption science: Removal rate = k 2 θ The net growth rate: UNDERSTANDING FOULING INDUCTION PERIOD Further development of the model of fouling induction period

THE MODEL OF FOULING INDUCTION PERIOD The surface coverage model: The induction time defined as the time when dθ/dt reaches its maximum, that is θ = 0.5 θ max Effect of velocity can then be interpreted as follows: Effect of temperature:

MODEL APPLICATION - EFFECT OF TEMPERATURE ON INDUCTION TIME ▲ : surface temperature 69.8°C, t 0.5 = 10.2 hours. ■: surface temperature 75.7°C, t 0.5 = 7.4 hours. ♦: surface temperature 81.4°C, t 0.5 = 5.05 hours. Data after Augustin et. al. [4]. Proceedings of the 7th International conference of Heat Exchanger Fouling and Cleaning, 2007, pp

MODEL APPLICATION - EFFECT OF VELOCITY ON INDUCTION TIME Model fittings for Mwaba et al. data From left to right: 0.3 m/s, 0.6 m/s, 1.0 m/s Symbols: Mwaba experimental data; Lines: model fittings Data after Mwaba et al. Heat Transfer Engineering, 27: 3, (2006), 42 – 54.

STIRRED CELL FOR EXPERIMENTAL FOULING INVESTIGATION Task 1.1. A compact fouling test cell based on Eaton and Lux’s patent is available for fouling tests Can operate at pressure up to 30bar and temperature up to 400°C Requires a small volume of sample (1 L) Easy to operate and change operational conditions. The test surface can be modifies with coatings or fins

FOULING CURVE OF CRUDE A ON WIRED PROBE Test condition: Bulk temperature 260°C; Surface temperature 399 °C Stirring speed 160 rpm, heat flux 79 kW/m 2

FOULING RATE COMPARISON – BARE PROBE VS WIRED PROBE Effect of wires on fouling ProbeSurface temperature (°C) Fouling rate (m 2 K/J ) Bare probe fouling x Wired probe no fouling Wired probe fouling x Bulk temperature 258 °C; Stirring speed: 160 rpm The existence of wires shows mitigating effect on fouling

EXPERIMENTS OF CaSO 4 FOULING Copper probe; Bulk temperature 55°C; Stirring speed 160 rpm

CaSO 4 FOULING RATES ON DIFFIRENT SURFACE Surface material/ condition Mild steel wired (Cal Gavin) Copper Stainless steel Surface Temperature (ºC) Fouling rate (m 2 K/kJ) 4.7E-63.3E-61.33E-58.3E-7 Bulk temperature 55°C; Stirring speed rpm The wires attached to the surface alternate the flow mode and have a mitigation effect on fouling

Develop CFD models on the updated software platform, Comsol 4.1 To Solve Velocity and shear stress distributions in stirred cell Heat transfer in tube with inserts CDF SIMULATION FOR FLUID FLOW IN THE STIRRED CELL WITH WIRED PROBE AND TUBE WITH hiTRAN INSERTS Progress in Task 1.2.

k-ε model – basic equations Equation of continuity Equation of momentum Equations of turbulent kinetic energy (k) and dissipation rate of turbulent energy (ε): CFD SIMULATION USING COMSOL SOFTWARE

CFD RESULTS - VELOCITY DISTRIBUTION in the stirred cell with wired probe

CFD SIMULATION FOR FLUID FLOW IN STIRRED CELL Comparison of shear stress over the probe surface – around a circle (0 - 2 π) Stirring speed 200 rpm; Bulk temperature 260ºC

COMPARISON OF THE SHEAR STRESS IN FRONT AND BEHIND THE WIRE Arrow: flow direction; Stirring speed 200 rpm; Bulk temperature 260ºC

EQUIVALENT TUBE FLOW RE NUMBER FOR SWIRL FLOW IN THE STIRRED CELL WITH WIRED PROBE The equivalent Re would allow the fouling data obtained using the stirred cell to be useful in the cases of tube flow

hITRAN BY CAL GAVIN

RECAP WALL SHEAR STRESS DISTRIBUTION Shear stress data are obtained from the velocity gradient and the turbulent viscosity by CFD simulation Z position begins at just behind the loop edge, ends at the same position of the next loop

TEMPERATURE FIERLD OF FLUID FLOW IN A TUBE WITH hiTRAN INSERTS Horizontal slice Vertical slice Temperature scale (K) Insert section, Surface temperature T s Post-insert section. Thermal insulation Pre-insert section. Thermal insulation

METHOD FOR CALCULATION OF AVERAGE HEAT TRANSFER COEFFICIENT The heat is gained by a small portion of fluid in an annulus of diameter r, thickness ∆r, and unit height passes a distance L from the bottom to the top

METHOD FOR CALCULATION OF AVERAGE HEAT TRANSFER COEFFICIENT The heat obtained by the fluid contained in a cylinder of radius R and unit height is given by: The average heat transfer coefficient can then be calculated as follows: T s and T b are the temperatures at the surface and in the bulk fluid, respectively.

AVERAGE HEAT TRANSFER COEFFICIENT FOR TUBE FITTED WITH INSERT Velocity (m/s) h Bare tube Experimental [Phillips 1999] h Tube with inserts Experimental [Phillips 1999] h Tube with insert by simulation The inserts significantly enhance heat transfer Phillips D, 1999, PhD thesis, University of Bath

CONCLUSIONS AND FURTHER WORK The further developed model for fouling induction period is capable of interpreting the effects of velocity as well as temperature on fouling induction time. Modification of the geometry of the probe surface with wire attachment shows a mitigating effect on fouling, which can be interpreted by the intensified turbulence revealed by CFD simulation. Further CFD work will be carried out for heat transfer in the stirred cell with modified probe. The proposed method with the help of CFD simulation offers a practical solution for estimation of average heat transfer coefficient for tube fitted with hiTRAN inserts, revealing a significantly enhancement of heat transfer by the inserts. Crystallization fouling can be carried out using the stirred cell. Further experiments will be arranged for investigation of the influence of surface properties on fouling.