The MWNFs were produced using a two-step synthetic method.
Stabilizers affect the thermal properties and related physical and chemical properties of MWNFs [5, 6].
First, we selected MWNFs with higher stability by using the static position method.
Because of the opacity of the MWNFs, [V.sub.z] could not be directly measured using the dynamic light scattering (DLS) method.
The smaller viscosity differences between nanofluids (low thixotropic or rheopectic properties) in the test process may indicate that the structure of the nanofluids showed little damage or that the nanofluids exhibited structural recovery, which suggested higher stability of the MWNFs.
MWNFs with different concentrations of MWCNTs were produced with the optimal type and fixed concentration ratio of stabilizers, and their stability, thermal conductivity, and pH were tested to confirm the usefulness of the selected stabilizer for heat transfer applications.
The UV/VIS/NIR spectrometer was used with the static positioning and centrifugal sedimentation methods to determine the difference between the absorbance of MWNFs initially ([ABS.sub.i]) and at a later test time ([ABS.sub.t]).
The results were used to determine the shear rate dependence of the thixotropic or rheopectic properties, and the shear rate dependence in (2) was used for evaluating the stability of the MWNFs. A low sum of viscosity differences (DRT) represents higher stability of MWNFs and vice versa:
The results were applied to (3) to determine the time dependence of thixotropic or rheopectic properties and to evaluate the stability of the MWNFs. A low relative standard deviation (RSD) of viscosity represents higher stability of MWNFs and vice versa:
High viscosity results in a pressure drop in the pipeline in the heat exchange system; therefore, AG is not suitable for producing MWNFs that can be used as heat exchange working fluids.
Table 3 lists the basic characteristics of MWNFs with 0.5 wt% MWCNTs; the MWNFs contain different stabilizers and stabilizer concentrations.
Figure 3 shows a photograph of the static position experiment after 30 days; MWNFs with different stabilizers and concentrations were used in the experiment.