Hydrodynamic effects on soiled surfaces : an experimental study and theoretical analysis

Abstract

This thesis presents the findings of an investigation aimed at understanding the effects of hydrodynamic shear stress on soil removal from textile surfaces both inside the washing machine and in closely controlled laboratory conditions. The research has involved developing a technique for the indirect measurement of shear stress based on pressure recordings and small block probes that provide both an indication of shear stress magnitude and direction. The probes have been investigated and calibrated in a purpose-built, rectangular-section, water tunnel in which tests were also carried out on specially prepared soiled cotton textile samples. This has allowed the correlation between shear stress and soil removal efficiency for 15k ≤ Re ≤ 155k. A more general investigation involving the washing machine has also been carried out so as to quantify and compare the effects of detergency, water temperature, time, abrasion, warping and tangential shear force. The hydrodynamic conditions inside the washing machine were investigated providing insight into flow conditions both on the inside and outside of the wash load. This was achieved through the use of a wireless device in the form of a sphere with an on-board pressure-flow sensor and radio transmitter for remote monitoring via radio. This remote data acquisition system was designed, developed and patented by the author. A model of the wash load motion has also been developed and high speed filming techniques employed to qualify and quantify the wash load dynamics. The main outcome of the research may be summarised as follows: - Hydrodynamic shear stress up to 7.7Pa a circa (15k ≤ Re ≤ 155k) is inadequate to remove soil from the standard EMPA textile surfaces. - Only 10-15% of the soil removed in the washing machines can be attributed to abrasion, warping (due to churning) and hydrodynamic shear stress, the rest is attributed to detergency and heat transfer. - There are at least two different flow domains within a horizontal-axis washing machine, one on the inside of the wash load and one on the outside. - Flow conditions on both the inside and outside are turbulent and velocities up to several meters per second have been recorded