Slip ratio (or velocity ratio) in gas–liquid (two-phase) flow, is defined as the ratio of the velocity of the gas phase to the velocity of the liquid phase.
In the homogeneous model of
two-phase flow
In fluid mechanics, two-phase flow is a flow of gas and liquid — a particular example of multiphase flow. Two-phase flow can occur in various forms, such as flows transitioning from pure liquid to vapor as a result of external heating, separ ...
, the slip ratio is by definition assumed to be unity (no slip). It is however experimentally observed that the velocity of the gas and liquid phases can be significantly different, depending on the flow pattern (e.g. plug flow, annular flow, bubble flow, stratified flow, slug flow, churn flow). The models that account for the existence of the slip are called "separated flow models".
The following identities can be written using the interrelated definitions:
:
where:
* S – slip ratio, dimensionless
* indices G and L refer to the gas and the liquid phase, respectively
* u – velocity, m/s
* U –
superficial velocity Superficial velocity (or superficial flow velocity), in engineering of multiphase flows and flows in porous media, is a hypothetical (artificial) flow velocity calculated as if the given phase or fluid were the only one flowing or present in a give ...
, m/s
*
–
void fraction, dimensionless
* ρ – density of a phase, kg/m
3
* x –
steam quality
Steam is a substance containing water in the gas phase, and sometimes also an aerosol of liquid water droplets, or air. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporizatio ...
, dimensionless.
Correlations for the slip ratio
There are a number of correlations for slip ratio.
For homogeneous flow, S = 1 (i.e. there is no slip).
The Chisholm correlation is:
The Chisholm correlation is based on application of the simple annular flow model and equates the frictional pressure drops in the liquid and the gas phase.
The slip ratio for two-phase cross-flow horizontal tube bundles may be determined using the following correlation:
where the Richardson and capillary numbers are defined as
and
.
For enhanced surfaces bundles the slip ratio can be defined as:
Where:
* S – slip ratio, dimensionless
* P – tube centerline pitch
* D – tube diameter
* Subscript
– liquid phase
* Subscript
– gas phase
* g– gravitational acceleration
*
– minimum distance between the tubes
* G-mass flux (mass flow per unit area)
*
– dynamic viscosity
*
– surface tension
*
– thermodynamic quality
*
– void fraction
References
{{DEFAULTSORT:Slip ratio (gas-liquid flow)
Fluid dynamics