INTENSIFIED HEAT TRANSFER TECHNOLOGIES FOR ENHANCED HEAT RECOVERY

Project meetingJuly 8, 2011

Veszprem, Hungary

WP1 - Enhancing understanding of heat exchange under fouling

Professor Barry Crittenden andDr Mengyan YangUniversity of Bath

01E102 rf

0

0.005

0.01

0.015

0.02

0.025

0.03

01E102 rf

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 CaSO4 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 1st order to θ, Hence: Growth rate = k1θ(1-θ)

• Further development - Adopting the concept of removal from the surface as in adsorption science: Removal rate = k2θ

• The net growth rate:

'Rdt

dR f

UNDERSTANDING FOULING INDUCTION PERIOD

Further development of the model of fouling induction period

21 )1( kk

dt

d

tkkeck

kk)(

1

21

211

1

THE MODEL OF FOULING INDUCTION PERIOD

215.0

ln

kk

ct

The surface coverage model:

The induction time defined as the time when dθ/dt reaches its maximum, that is θ = 0.5 θmax

8.02 uk

Effect of velocity can then be interpreted as follows:

RT

EAct ii

ln)ln(lnln 5.0Effect of temperature:

MODEL APPLICATION - EFFECT OF TEMPERATURE ON INDUCTION TIME

0

0.0001

0.0002

0.0003

0.0004

0.0005

0 5 10 15

Time (hour)

Rf (

Km

2/k

W)

▲: surface temperature 69.8°C, t0.5 = 10.2 hours. ■: surface temperature 75.7°C, t0.5 = 7.4 hours.

♦: surface temperature 81.4°C, t0.5 = 5.05 hours.

Data after Augustin et. al. [4]. Proceedings of the 7th International conference of Heat Exchanger

Fouling and Cleaning, 2007, pp. 206-21

0

2

4

6

8

10

12

14

0 2000 4000 6000

Time (min)

Fo

ulin

g r

ate

(x10

-4 m

2K

/W)

MODEL APPLICATION - EFFECT OF VELOCITY ON INDUCTION TIME

Model fittings for Mwaba et al. dataFrom 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.

230

mm

app

roxi

mat

ely

t wb t ws

t bulk

INS

ULA

TIO

N

INS

ULA

TIO

N

© 2008 University of Bath, England

t wm

Fil Level

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

0

t

bs

t

bsf q

TT

q

TTR

q

TTR sostf

Test condition: Bulk temperature 260°C; Surface temperature 399 °C

Stirring speed 160 rpm, heat flux 79 kW/m2

FOULING RATE COMPARISON – BARE PROBE VS WIRED PROBE

Effect of wires on fouling Probe Surface temperature

(°C)

Fouling rate

(m2K/J )

Bare probe fouling 383.7 1.10 x 10-9

Wired probe no fouling 385.5 -

Wired probe fouling 399.3 5.47 x 10-10

Bulk temperature 258 °C; Stirring speed: 160 rpm

The existence of wires shows mitigating effect on fouling

EXPERIMENTS OF CaSO4 FOULING

Copper probe; Bulk temperature 55°C; Stirring speed 160 rpm

CaSO4 FOULING RATES ON DIFFIRENT SURFACE

Surface material/condition

Mild steelMild steel

wired(Cal Gavin)

CopperStainless

steel

Surface Temperature

(ºC)95 98 89 79

Fouling rate(m2K/kJ)

4.7E-6 3.3E-6 1.33E-5 8.3E-7

Bulk temperature 55°C; Stirring speed 150 - 160 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

2T

Tk

T uukkut

k

k

CuukCCut

TT2

2

2

1

CFD RESULTS - VELOCITY DISTRIBUTIONin 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

0.00

10.00

20.00

30.00

40.00

50.00

60.00

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016

Position in z direction

Sh

ea

r s

tre

ss

(P

a)

0.5m/s 0.7m/s 1m/s 1.5m/s

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 Ts

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:

Ts and Tb are the temperatures at the surface and in the bulk fluid, respectively.

AVERAGE HEAT TRANSFER COEFFICIENT FOR TUBEFITTED WITH INSERT

Velocity (m/s)

hBare tube

Experimental [Phillips 1999]

hTube with inserts

Experimental [Phillips 1999]

hTube with insertby simulation

0.5 490 1460 1644

1.0 780 2150 2292

2.0 1280 3460 3768

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.