An apparatus for imaging the thermal properties of fluids in a wellbore. The apparatus includes a tool mandrel adapted to traverse the wellbore. The apparatus includes temperature sensors positioned at spaced apart locations along radially extendible arms attached to the mandrel. A temperature measuring circuit connected to each one of the sensors. The apparatus includes a circuit for applying a momentary current pulse to each of the temperature sensors, for momentarily raising their temperatures. Momentary increases in the sensor temperatures are provided so that the thermal transient response of the fluid in contact with each sensor can be determined. The invention includes a method of determining the flow regime of the fluid flowing in the wellbore. The method includes the steps of measuring the temperature of fluids in the wellbore at spaced apart locations within the cross-sectional area of the wellbore, using a tool having temperature sensors positioned at spaced apart locations. The flow regime is determined by generating a temperature map of the wellbore and comparing the temperature map with temperature maps of known flow regimes. The method includes the steps of applying current pulses to the temperature sensors to momentarily raise their temperature, measuring the temperature decay at each sensor, thereby determining the thermal transient response of the fluids in contact with each sensor. A map is generated of the thermal transient response and the thermal transient response map is compared to maps of transient responses in known fluid flow regimes.