J. Cent. South Univ. Technol. (2008) 15(s1): 326-328
DOI: 10.1007/s11771-008-373-4
Non-Newtonian steady shear flow characteristics of waxy crude oil
Huang Shu-xin(黄树新)1, Chen Xin(陈 鑫)1, Lu Chuan-jing(鲁传敬)1,
Hou Lei(侯 磊)2, FAN Yu-run(范毓润)3
(1. Department of Engineering Mechanics, Shanghai Jiaotong University, Shanghai 200240, China;
2. Department of Petroleum Storage and Transportation, China University of Petroleum, Beijing 102200, China;
3. State Key Laboratory of Fluid Power Transmission and Control, Zhejiang University,Hangzhou 310027, China)
Abstract: The experimental research on the non-Newtonian flow characteristic of a waxy crude oil was conducted through a rotational parallel-plates rheometer system. The test temperature is about 6.5 ℃ higher than its gel point. The shear stress and viscosity of the waxy crude oil show sophisticate non-Newtonian characteristics in the shear rate of 10-4-102 s-1, in which the shear stress can be divided into three parts qualitatively, i.e. stress-up region, leveling-off region, and stress-up region. This indicates that there is a yielding process in shearing for the waxy crude oil at the experimental temperature, which is similar to the yield phenomenon in thixotropy-loop test discussed by CHANG and BOGER. Furthermore, the steady shear experiment after the pre- shear process shows that the stress leveling-off region at low shear rate disappears for the waxy crude oil and the stress curve becomes a monotonic climbing one, which demonstrates that the internal structure property presenting through yielding stress at low shear rate can be changed by shearing. The experimental results also show that the internal structure of waxy crude oil presenting at low shear rate has no influence on the shear viscosity obtained at the shear rate higher than 0.1 s-1. The generalized Newtonian model is adopted to describe the shear-thinning viscosity property of the waxy crude oil at high shear rate.
Key words: waxy crude oil; shear flow; non-Newtonian flow; generalized Newtonian model
1 Introduction
The crude oil produced in China often has high waxy component[1]. The flow of waxy crude oil at low temperature shows sophisticated non-Newtonian characteristics, such as yield property[2] and thixotropy property[3], etc. It is a valuable work to research the non- Newtonian characteristics of waxy crude oil because of its application background.
HOU et al[4-6] have done some careful and extensive experimental research on the non-Newtonian characteristics of waxy crude oil, in which the complex influence of temperature and the changing rate of temperature on the viscoelasticity of waxy crude oil was included. CHANG and BOGER[2] studied the yield and thixotropy behavior of a waxy crude at the temperature lower about 13 ℃ than pour point. Recently, VISINTIN et al[7-8] have tested the viscoelastic behavior of waxy crude oil at different water cuts and different temperatures, as well as yield stress of the oil.
The experimental results approach to the real flow behavior of crude oil by considering the effects of shear, temperature, and others factors simultaneously. However, it becomes more difficult to understand and describe the viscoelasticity of waxy crude oil when considering all the effects. The shear effect on the non-Newtonian characteristic of a waxy crude oil was researched at a single temperature in the present paper. The viscosity properties without and with preshear were obtained, and the descriptions of three models on the viscosity at high shear rate were also conducted.
2 Experimental
The experimental waxy crude oil was Shengli crude oil, supplied by the Department of Petroleum Storage and Transportation, China University of Petroleum. The oil contained 7.85% wax, 17.96% resin, and 1.21% asphalt-tene, which were also supplied by China University of Petroleum. The gel point of the crude oil was 8 ℃ tested in terms of GB/T510-83(91).
Bohlin-Gemini-200 was used in the test, which was a stress-controlled rotational rheometer. A parallel-plates system with plate diameter of 40 mm was adopted in the experiments. The temperature controlled by an indoor
air-conditioner was 14.5 ℃, and the real temperature in tests shown by Gemini-200 rheometer was (14.5±0.3) ℃.
The flow characteristic of waxy crude oil is related to the structure and form of wax contained, which is influenced significantly by temperature history. Thus, a pre-process with high temperature is needed to eliminate the influence of the unknown thermal history[2, 5-6]. The pre-process adopted in this test was that the crude oil was put in a water bath apparatus at 80 ℃ for 2 h, and then was cooled at the indoor temperature of about 7 ℃.
The steady shear viscosity was tested under the rate- controlled condition in viscometry mode of Gemini-200, and the preshear was conducted using step shear rate mode at the rate-controlled condition. The gap between plates is 1.2 mm.
3 Results and analysis
The shear stress and the viscosity of the waxy crude oil tested at temperature about 6.5 ℃ higher than its gel point are shown in Fig.1, in which seven groups of data are given simultaneously. These data show good agreements although the test temperature has a slight change. Fig.1 indicates that the shear stress and the viscosity of the waxy crude oil show sophisticate non-Newtonian characteristics in the shear rate of 10-4- 102 s-1, in which the shear stress can be divided into three parts qualitatively. The first stress-up region occurs below point ‘a’; the leveling-off region occurs between points ‘a’ and ‘b’; and the second stress-up region occurs above point ‘b’. The shear rate is about 10-3 s-1 at point ‘a’, and 2×102 s-1 at point ‘b’. The stress in leveling-off region between 10-3 to 2×102 s-1 is about 0.35 Pa, which indicates that the waxy crude oil has a yielding property in shearing at the experimental temperature, caused by the micro-structure of wax in the crude oil. This result is similar to the yield phenomenon in thixotropy-loop test discussed by CHANG and BOGER[3].
Fig.1 Steady shear property of Shengli crude oil at 14.5 ℃
The viscosity curve becomes complicated also because of the existence of the micro-structure of wax, and the slope of the viscosity curve at low shear rate is different apparently with that at high shear rate. However, the experimental results (No.13) indicates that the internal structure of waxy crude oil presenting at low shear rate has no influence on the shear viscosity obtained at the shear rate higher than 0.1 s-1.
Fig.2 shows the effect of preshear on the steady shear property. The shear rate used for preshear is 0.1 s-1, and the time interval for preshear is 10 min or 15 min. The steady shear test starts after 6 min in the end of preshear. The distinctive feature in Fig.2 is that the stress leveling-off region at low shear rate disappears for the waxy crude oil and the stress curve becomes a monotonic climbing curve, which means that the internal structure property presenting through yielding stress at low shear rate can be changed by shearing, and the microstructure cannot be formed in short time interval. However, the results also present a slight high stress and a slight high viscosity in the shear rates of 0.1-10 s-1, which indicates that a new microstructure might be formed may the test.
Fig.2 Shearing property of Shengli crude oil at 14.5 ℃ after preshear
4 Model description on viscosity
The experimental results show that the internal structure of waxy crude oil presenting at low shear rate has no influence on the shear viscosity obtained at the shear rate higher than 0.1 s-1. In addition, the flow in engineering often occurs at high apparent shear rate. It should be a meaningful work in engineering to describe the viscosity property of the waxy crude oil at high shear rate. Therefore, three generalized Newtonian models used extensively in engineering, i.e. Power-law model, Carreau-A model, and Cross model, were tried to describe the viscosity property of the waxy crude oil in the experimental condition.
The constitutive equation of the generalized Newt- onian fluid can be expressed as
τ=2η(I2)d (1)
where t is the extra stress tensor; h is the viscosity; d is the rate of deformation tensor; I2 is the second invariant of shear rate. The power-law model used for describing the viscosity is
(2)
The Carreau-A model is
(3)
The Cross model is
(4)
where k, n, h0, l and a are the parameters of the three models.
The values of parameters of the three models were obtained by fitting the steady shear viscosity data of the Shengli crude oil at 14.5 ℃. The results are listed in Table 1. The calculated viscosities by the three models are shown in Fig.3.
Two groups of experimental data were fitted by Power-law model. One group contains all the experi-
Table 1 Parameters values describing shear viscosity of Shengli crude oil at 14.5 ℃
Fig.3 Experiment and calculations on steady shear viscosity of Shengli crude oil at 14.5 ℃
mental viscosity data in Fig.3, and the fitting results are denoted using the symbol ‘Powerlaw-all’. The other only contains the data at the shear rate higher than 1.5 s-1, and the calculated results are denoted by the symbol ‘Powerlaw-part’. The results show that Power-law model cannot fit all the data in the whole experimental shear rate, but it can describe the data at high shear rate. The steady shear viscosity of Shengli cruid oil at 14.5 ℃ can be well described by Carreau-A model. The fitting of cross model is inferior to that of Carreau-A model, but superior to that of Power-law model.
5 Conclusions
The experiments on the non-Newtonian flow characteristic of a waxy crude oil were conducted at the temperature about 6.5 ℃ higher than its gel point. The shear stress and the viscosity of the waxy crude oil show sophisticate non-Newtonian characteristics in the shear rates of 10-4-102 s-1. The waxy crude oil presents a yielding characteristic for stress at the temperature tested. The steady shear experiment after a pre-shear process indicates that the internal structure property presenting through yielding stress at low shear rate can be changed by shearing. Furthermore, the internal structure of waxy crude oil presenting at low shear rate has no influence on the shear viscosity test obtained at the shear rate higher than 0.1 s-1. The Carreau-A model was used to well describe the shear-thinning viscosity property of the waxy crude oil at high shear rate.
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(Edited by CHEN Wei-ping)
Foundation item: Project(07zr14047) supported by the Natural Science Foundation of Shanghai, China
Received date: 2008-06-25; Accepted date: 2008-08-05
Corresponding author: HUANG Shu-xin, PhD; Tel: +86-21-54745679; E-mail: huangshuxin_2001@sina.com
